The difference between men and boys is the size and number of their toys, a mean and perceptive lady once said. My latest toy is a tree house.

I dignify it as a "wildlife observation platform," but not even my young son is fooled. He knows the reason for building such an elaborate thing up in a tree is that it's fun.

It is a 4 x 8-foot plywood deck, framed, reinforced and railed with 2 x 4s mounted atop a tulip poplar cut off 40 feet above the ground. The tree was decapitated reluctantly but necessarily, since the swaying of untopped trunks in the wind eventually will loosen or crush any platform mounted in high forks; my father died from having one he had built for me collapse under him.

A poplar was chosen because the species is remarkably tolerant of pruning and injury. The remaining limbs and trunks quickly sprouted new leaves in response to the sunlight that now reaches them.

The location was suggested by game trails, for the fascination of being comfortably up a tree is watching the unsuspecting wildlife below. The poplar, overlooking several branches of a draw, is in such a hot spot that deer came snorting around its base while I was clattering away above, and a doe has chosen the tangle of trimmed branches as a place to bed her fawn.

A bonus has been no end of birds. Several arboreal species I never could confirm from the ground with binoculars have been added to my life list from eyeball ranges of as little as 10 feet. Bald eagles and ospreys have been swooping around as though checking it out for a nest site. They are welcome to it.

Mindful of my father, and of my children, I spent a lot of time thinking how to make the rig ridiculously strong and impossible to fall out of. Being myself more than a little afraid of heights, I prefabricated the thing in the garage to mimimize the time that would have to be spent mounting it on the tree. Each piece was cut, fitted, drilled, bolted and labeled and then it was all knocked down. It was well worth the trouble: Little problems that showed up in the garage would have meant hours of frustration up in the tree.

Nails were used only as temporary pins because they tend to rust out or work loose. Plated carriage bolts hold the structure together, lag bolts lock it to the tree, and the whole affair is wrapped with fence wire. It cost about $150, which should come to less than $15 a year over its useful life.

Forty feet is awful high, but the placement of such a platform is dictated by the shape of the tree. This one went on a triple fork, which provided a stable triangular base with a minimum of substructure. Topping the tree made it unnecessary to adapt the platform to fit among the trunks, which would have cost both strength usable space.

Access is by spikes driven every 18 inches up to the first fork and then by a ladder made of saplings wired between the two main trunks. An 18 x 24-inch trap-door was cut through the 3/4-inch plywood floor; when in place it rests on a frame of 2 x 4s that are supported by the substructure. Because it's framed, the trap-door does not weaken the plywood, and entry from underneath avoids a dangerous climb over the outside.

The first sunny day and rainy night spent in the tree showed that some sort of canopy was needed. A 10 x 10-foot ripstop nylon tarp, supported by tent poles mounted in flagpole holders, serves beautifully. It can be rigged or struck in two minutes. Since the long axis of the platform is north-south, the canopy provides the most shade where it's most needed, east and west. It has survived a gale that knocked down nearby trees.

Because of its design the platform can be mounted in almost any substantial tree or on posts for a backyard playhouse. It's stout enough to be reassuringly rigid even if not evenly supported (it can be cantilevered nearly half its length).

The key to the structure is the 2 x 4s, overlapped at the corners, that sandwich the rim of the plywood sheet. This not only compresses the edges of the plywood to prevent wraping and separation of layers but yields a frame much stronger -- and cheaper -- than solid lumber of the same dimensions. The railing and deck lock together into a truss whose parts are under both tension and compression and so lend strength to each other.

Since it isn't possible to build a tree house without injuring the tree, it makes sense to cut a few large branches or trunks rather than inflict a lot of lesser wounds. If the platform is to be mounted between trunks that are not cut off, they should be as thick and as low on the tree as possible, because even the thickest untopped trunks will move a little.

If a suitable tree is lacking, the deck can be mounted on six 4 x 4 posts in concrete footings; the space beneath is ideal for a playhouse or a tool shed. If the posts are not enclosed or cross-braced, use 6 x 6s.

No fancy carpentry or power tools are required. The tools needed are a hand saw, a hammer, a brace (hand drill) with 1/4, 3/8 and 1/2-inch bits, an adjustable wrench, an eight-foot rule, a keyhole saw and pliers.

Preassembly and knockdown should take about half a day; reassembly in the tree will take as long as it takes. MATERIALS 1 -- 4 x 8 x 3/4 AC plywood (finished on one side). If the platform is to be supported at all corners, 1/2-inch plywood will do. Check the sheet carefully; they sell an awful lot of awful plywood these days. 13 -- 2 x 4 8 pressure-treated studs, plus as many more as are required for the subframe; this will vary according to the tree. If the span between trunks is more than five feet, double the subframe members. Accept only straight, sound boards without big knots. 1 -- gallon of Cuprinol or an equivalent wood preservative. 8 -- 8 x 3/4-inch plated lag bolts, plus as many more as will be needed (two to each member) to bolt the subframe to the tree. 4 -- 4 x 1/4-inch plated lag bolts for fastening the platform to the subframe. If the deck is to be cantilevered, use 8 x 1/4-inch plated carriage bolts, which will go through the 4-inch thickness of the sandwich and the 3 5/8 inches of a subframe "2 x 4." 8 -- 3 x 1/4-inch plated carriage bolts (if needed to frame the trap door; study the directions and the tree before shopping). 8 -- 4 x 1/4-inch plated carriage bolts. 24 -- 4 1/2 x 1/4-inch plated carriage bolts. 12 -- 6 x 1/4-inch plated carriage bolts. 22 -- 16-penny nails. 1 pound of chicken-wire staples. 1 piece, 4 x 25 feet, of heavily galvanized wire fencing. The lighter the wire the easier to handle (but chicken wire is too light). Make sure the openings are too small for kids to stick their feet into, or they'll climb on it and ruin it no matter how many times you tell'em no to. An alternative is aluminum chain-link fencing, which is heavy but fairly easy to handle and lasts forever, in the modern sense of that word. It will require the same weight of the next size larger staples.

One washer will be needed for each lag bolt and two for each carriage bolt, to keep the heads and nuts from sinking into the wood. METHOD Cut four of the 2 x 4s to 80 inches. Cut two 44-inch sections from another one. From each of two more 2 x 4s, cut 44-inch pieces. These are the eight "sandwich" pieces. The two 50-inch sections left over will become the end pieces of the railing. Sometimes treated studs will run longer than eight feet; if you can come up with two 52-inch (or longer) pieces, so much the better.

If the platform is not to be disassembled, the plywood should be treated with preservative before the boards are bolted. Otherwise, wait until after disassembly; so that the preservative will penetrate the bolt holes.

Arrange two long and two short pieces, wide sides up, into a 4 x 8 rectangle. Lay the plywood on them, finished side up, and arrange the rest of the cut boards on top of the sheet, staggered so that the gaps at the corners of the upper and lower layers do not coincide. The boards won't quite meet; the gaps are for dripstops and drainage.

Make sure all the 2 x 4s are flush with the edges of the sheet and fasten them with 16-penny nails, driven flush. Make sure that each 2 x 4 is pinned in at least two places, but no closer than nine inches from any corner. The big nails are necessary because the platform must be manipulated for drilling and bolting, and shorter nails may pull loose.

Set the platform on blocks or sawhorses. Drill a 1/4-inch hole through the center of each corner and install 4 1/2-inch carriage bolts, heads up. All bolts should be tightened until the washers press into the wood but not so tight it begins to crush. Set another bolt five inches in each direction from each corner bolt. This will make three bolts at each corner and locks athe ends of the 2 x 4s. Set bolts in the center of both ends and sides. Space four more evenly along each long side, for a total of 24.

Cut three 2 x 4s in half for railposts. It doesn't matter whether they are exactly 48 inches but they must be equal.

Set a railpost at the end of one of the long sides, base flush with the bottom of the lower sandwich. If you don't have a helper to hold it in place, use one nail. Drill a 3/8-inch hole, about six inches deep, through the post and into the upper 2 x 4 of the sandwich. The hole will be parallel to the narrow end of the platform and will have to be off-center to miss the corner bolt. Drill another into the lower layer of the sandwich, off-center the other way. Insert 8-inch lag bolts. Repeat at each corner. All posts should be attached to the long sides of the deck.

Set a railpost at the center of one long side. Drill 1/4-inch holes, offset to avoid the center sandwich bolt, through the upper and lower 2 x 4s. It doesn't matter whether the middle post is exactly centered, but it should be set so that its bolts are on either side of the sandwich bolt. Fasten with 6-inch bolts. Mount the other middle post.

Install the side (long) rails, narrow side up and flush with the tops of the posts. Use one 4-inch bolt at each end and two at the middle posts, nuts outboard.

It the end posts are vertical, the span will be 54 1/2 inches between the outer edges of the ends of the side rails. Your end rail pieces probably aren't that long. So loop ropes between the side rails near each end and tighten them, bending the posts inward until the end rails overlap the ends of the side rails. Fasten each end rail with two pairs of 6-inch bolts through the end posts.

The platform now is finished except for the trapdoor and fencing, which come after the thing is mounted in the tree.

Before disassembling, label each piece by both position and orientation. Be thorough; you won't have a free hand for scratching your head when you're reasembling it in the tree. After it's apart, apply the preservative to the playwood, with particular emphasis on the edges, holes and top side. I put a coat on the 2 x 4s too, to get an even color.

Ask some friends over when you're ready to mount it. The plywood is heavy and clumsy to handle, and you'll want somebody there so you won't have to climb down for everything you drop or forget.

Placement of the supports will vary with the tree. The broader the bass of support and the more centered the placement of the platform on them, the better.

Don't cut the trap door until you're sure you know exactly where you want it. Then mark trhe corners and drill 1/2-inch holes to start the keyhole sawcuts. If you're lucky, it will be possible to rest the 2 x 4 "keepers" on the subframe. Otherwise, fasten the framing of the trapdoor to the deck with 3-inch bolts, two to each piece. The framing should protrude into the opening about 1/2 inch. After cutting, treat all eges with preservative. No handle or hinge is needed; although the trap door lies perfectly flush, it can be lifted easily by inserting a finger in a corner drill hole.

If any protruding bolt ends seem likely to snag somebody, cut them off flush with the nuts with a hacksaw.

Align the fencing parallel to, but 1/2-inch below, the outside top edge of the railing. If it hangs below the bottom of the platform, bend it under and staple. Staple each section of wire that lies over wood; the more places it is fastened the stronger and less stretchy it will be. The wire also stiffens and strengthens the framework by distributing stress.

The platform is big enough for several kids to sleep on, or an adult and a child, or two very friendly adults.