A flower. In light, it would grow, in dark it wuld wither. It seems simple enough behavior that something as versatile as the handy board would easily be able to imitate it.

Designing such a structure with legos turned out to be a challenge, however. A flower as described would have all vertical motion, and with gravity as well as friction working against the mechanism, it proved difficult to make it work.

The original plan was: A flower pot to hold the mechanism and the handy board, with a stem that grew up and then a flower that extended above. Leaves were a potential addition later on. There were some problems associated with this design though; in order for the flower to grow and then bloom, the mechanism for blooming would have to be raised up by the stem. I was concerned that the motors controlling the stem would have difficulty already, and adding weight to them would probably increase the trouble. Also, in order to conceal such a thing inside the stem, the stem would have to be quite wide -- certainly wide enough to make the number of legos needed to construct a suitable flowerpot prohibitive.

So I modified the design to only perform one raising: First the flower would be pushed through, then the stem would follow. Upon consideration, this effect seemed as difficult to produce as the original one, so I abandoned that idea and returned to the first. The flowerpot would simply have to be constructed out of something else. Eventually, for that purpose, I settled on a wire frame covered with cloth. A similar construction would be used for the stem. This pot had an advantage of being easily detachable from the rest of the device, so I would not have to worry about deconstructing a large thing every time I wanted to fix the gears.

The gearing was the most difficult part -- or rather, spacing and supporting the gear structure that worked. Eventually though, I had something that at least approximated what I'd envisioned. I used three light sensors to activate the robot, mostly for error control in case a particular sensor was more sensitive than another. A flashlight was sufficiently brighter than the ambient light to act as the 'sun', though flash photography would also trigger the flower to bloom.