This video has several interesting toy robots. Check at 7:00 when a bot draws by leaving a trail of sand. Center hole looks similar to the Scribblers which could probably do this too. Although probably necessary to drive in reverse, so the tail wheel doesn't drive through the sand.
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ps. Erco, Liberty has been working with her S2 this past week and with very little help from me, she has almost perfected her figure 8.
Now we're cooking! Need Whit onboard here too. Everyone except me should have sand all over their floor.
Then you'll need a Roomba in hot pursuit.
Andrew, how old is Liberty?