If nothing else, give Laser Pegs credit for being a really good idea. In fact, they’re actually a truly incredible idea: This set of modular construction pieces light up internally when connected to a power source.

If only the implementation worked as well as the concept.

These bastard children of Legos and Lite-Brite are bigger than you think, about the size of a typical Tinkertoy block. Made of clear plastic, each contains a colored LED. When connected to the included power source, a circuit is completed, and the Laser Pegs light up like Christmas lights. Just keep adding pieces and your glowing construction is automatically illuminated.

Tinker Toys are an apt comparison: Remember how hard it could be to wedge a Tinker Toy rod into the receiving hole in a block? Laser Pegs are almost as tricky. They’re (obviously) made of plastic, so variable drill hole sizes aren’t the issue, but the connections are tight nonetheless — they have to be to make the wiring work. There are several different shapes in the kit.

I put Laser Pegs in front of my kids: a 5-year-old boy and 8-year-old girl, though technically these are intended for ages 7 and up. They often struggled getting the pieces together, especially when multiple connections to a single block were involved.

Once their constructions were complete — guns, spaceships and lightsabers were, of course, the most popular concepts — Laser Pegs revealed more trouble.

Those connections which seem so tight when you just have two pieces to deal with turn out to be extremely rickety when you string a dozen or more together. Larger constructions start to bend and sway at the connecting points, and this makes the electrical connections built into the joints extremely temperamental. Breathe funny and the lights will go out on half your design until you jiggle it back together just so.

That’s a bummer, because it means that unlike Legos, kids can’t really play with their finished creations. They have to lay them on the ground and admire them from afar lest their lighting fail and the waterworks begin. If your house is the look-don’t-touch type where the furniture is wrapped in plastic, well, they’ll probably fit right in.

WIRED Kid verdict: “They’re pretty good.” Reasonably inexpensive.

TIRED Designs tend to be very flat. Extremely temperamental connections sap the fun out of things. Wire connecting battery pack to blocks is too short.

Photos: 1. Courtesy Laser Pegs. 2. Christopher Null/Wired.com