Okay, you want cool, here's cool: A technology that solves one of the most intractable problems of daddyhood. You know you should spend more floor time with your kid. The problem is: What can you do seated on the hardwood that captures the imagination of both a fourth-grader and--here's the hard part--a daddy with the attention span of a pre-schooler?

The answer surprised the heck out of me. It's the two new Lego Mindstorms robot kits for 9-year-olds and up.

I know, I know. These would seem to have all the elements for daddy/kid disaster. A gazillion little pieces of plastic just looking to be lost and ground up beneath bare feet. A final robotic outcome so complex that it seems guaranteed to end with a frustrated daddy throwing a baffling half-finished assembly at the wall, while the kid runs away in tears. And every kit comes with its own little computer that has to be mastered for the 'bot to do anything interesting.

Sounds like misery with AA batteries, right?

Well, turns out our friends at Lego have an astonishing little achievement on their hands. My daughter squirmed like a puppy at the very idea that she could create a robotic insect that would drive the cats insane. She saw the possibilities right away just by looking at the box. Then she saw she could actually build R2D2! Suddenly I was being grabbed by the hand and dragged up the stairs.

The insanely great part about these kits is not the 'bots. Anybody with a platoon of MIT grad students can build a robot kit. What Lego has done is demonstrate a deep understanding of kids and daddies.

The central brilliance of these kits is the instruction books. The very first thing they do, right after showing the child where to put the batteries, is persuade her to snap some motors and sensors onto the ittle bitty computer such that within seconds, all kinds of things start going whiz bang.

Immediate giggle response from both kid and daddy.

These are people with a clear grasp of the limits of delayed gratification.

Then comes the building of the 'bot around those things that go whiz bang. In the instruction book, no words are used to explain how to do this. Just drawings. Exceptionally intelligent, step-by-step, logical, painless, clear, easy drawings. Oh, I see. You stick this into that. Snap a coupla these over here.

Hours later, you and the kid find yourself still on the floor, going, "Oh wow, look at what this does. Oh cool, you can make it do that!"

The payoff for the kid, apparently, is building something that does what the kid tells it to do. Oh sure, there are a lot of things that you can put under a Christmas tree that will come with a remote. But that's not the same as being its creator. The idea that you actually built this thing that then does tricks when, for instance, you signal it with a flashlight beam--that's box office.

And the fact that you, the kid, can then disassemble it and reassemble it and make it look like any kind of weird thing you want--that's a real crowd pleaser. Then, there's the promise that you can actually create original programs for it, if you get far enough into that. Well, this makes you a master of the universe. Even that epitome of cool, my elder, 14-year-old daughter, was intrigued enough to want to muscle in.

The thrill for the daddy is that if you ever loved an Erector set, this whole Lego thing is immersive. You and the child poring over the illustrations, looking for the right pieces. Discovering that the kid is better at finding the right piece than you. Discovering that the kid's little hands are better at snapping together small widgets than you. You start getting mildly competitive with the kid. Well, if you can do that, watch me do this. Hours fly by. The sun is setting. Where did the time go? We're not done yet! We're going to have to come back to this tomorrow! (Cool!)

There are some things you should know about these kits:

First, if you go out to buy a pack of chewing gum to use to stick pieces of the 'bots together, it generally means that you haven't followed the instructions correctly, and you need to go back a few pages to find the step you must have skipped. Although there's a lot to be said for you and the kid sitting on the floor chewing gum together.

Second, you really want to do this on a nice big clear desk. Unless you're a practicing yoga instructor, sitting on the floor for as long as these kits will hold your interest is just hell on the knees.

And, finally, these kits may produce unexpected results. Like your daughter dusting off an old box that hasn't been touched for years and saying, "Daddy, will you play chess with me?"

The Lego Mindstorms Droid Developer Kit has a suggested retail price of $99, which seems to be pretty close to the street price; the Lego Mindstorms Robotics Discovery Set goes for $149.

CAPTION: Evie Garreau and her dad, Joel, plug in the "brains" of their Lego Mindstorms insect robot kit. Already completed on the table is their R2D2 kit.