Well, this will show you how to make one that will not fall! ever! :D
A bristlebot is very easy to make and will skate over any flat surface when done.
A bristlebot is also tons of fun for kids.

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what size and type of nails do you recommend or does it not matter since they are only to balance it out?
love it
so i thought i could improve it, and i tried some different stuff, and the nails worked best :)
sure that would work, you could try it! :) id love to see it!
and yeah, it would clean, a little. good luck! ;)
is there really need of the toothbrush??
Now, if you were to somehow attach bristles or otherwise rough-up the two back facing nails, and smooth the front one, you might get some motion... interesting notion...