Things are not always what they seem and on Friday they shouldn't be. April Fool's Day is a day for masquerade, a time to tease the eye and tease your friends, to celebrate fools and foolishness.

If you have the skill and the energy, you can do it with a dinner made up of tromp l'oeil foods:

Butter carved to look like flowers; radish roses; bread baked into a variety of shapes; grapefruits or oranges hollowed out and filled with lemon, raspberry or orange sherbet; a cake iced like a checkerboard, or baked in a three-dimensional mold so that it looks like a bunny or a lamb. (Kitchen Bazaar, 4455 Connecticut Ave. NW, and Seven Corners Shopping Center has the latter molds at $8 each.)

A copy of Judith Olney's book, Entertainments (Barron's, $24.95), will give you more ideas on how to prepare and serve food to fool the eye.

If that's too much work, serve the meal in dishes that masquerade as something else. For example, Little Caledonia (1419 Wisconsin Ave. NW) has a ceramic pig, debonair with a bottle of port tucked under its arm, which is really a pitcher; its snout's the spout.

For the guest bathroom, there are soaps that look like everything but: You can wash your hands with crayons, available at a number of shops. Gifts, Unltd., 1617 Connecticut Ave. NW also has soaps disguised as M&M's ($4), chocolate-chip cookies ($3.50) or popsicles ($3).

A box of chocolates is realistic enough to need a warning that they are to be used for washing--and not eating--up ($9.25 for a box of 12). A wicked party-giver might be tempted to offer them with the after-dinner coffee.

The Coffee Connection, 1627 Connecticut Ave. NW, has wooden eggs for 95 cents each or $8.25 a dozen. Properly painted, they could fool the guests on April 1 and the Easter Bunny April 3.

Even the chef need not be what he or she seems:

Walpole's, 1722 Connecticut Ave. NW, has an apron that makes the chef look as though he's wearing black tie ($25). The Cook's Connection, 1803 18th St. NW, has one that masquerades as a chorus girl's lacy lingerie, complete with dangling garters ($39).

There is still time to invite people to a costume party where everyone wears masks, and no one lets on who they really are until midnight, when the clock chimes the end of April Fool's Day. People can come as famous fools (a court jester, for instance, or someone whose foolishness cost them dearly, like Marie Antoinette). Or foolers (the Trojan horse or P.T. Barnum).

Al's Magic Shop, 1012 Vermont Ave. NW, is an old hand at fooling people and probably the only store in Washington that has a sign proudly announcing, "Yes, we have warts!" (Along with everything else, from moustaches to makeup.)

To entertain your guests appropriately on this day of fools, take their pictures with a camera that squirts water ($1.50 and $2.50), put a phony faucet in the bathroom ($2.50) where they will go to wash off the vampire blood ($1.25) that suddenly appears on their necks.

Use a lighter that squirts water (49 cents) and nail a nickel to the floor ($1) and then, just as everyone is about to stalk out of your house in disgust, wave your arms and scream "APRIL FOOL!"