Curiosity, and a desire to figure out how things work.
[Notice the boys work at it, until "HOW" is figured out.]
To this day, Maxwell will periodically jump up onto the bookcase where the Newton's Cradle is stored...and the "clack-clack-clack" of balls being struck is heard throughout the house as he starts it up again.
Brilliant. (Not that we're prejudiced or anything. Nope. Not one bit.)
Happy National Engineer's Week!