If you love building with Legos, you might want to also try Fischer Technik, the German-made super quality building toy. A lot of the pieces are put together with dove-tail slides, so the end assembly is very rigid. I stumbled upon the Hobbyengineering.com website ordered a kit and now my kids are junkies. I have build with many different construction toys: Erector Set, Legos, Construx, K'nex. They all have their own appeal, but Fischer Technik is fascinating even for an adult.