That's probably why they never show it on sand - it would suck it up. Things with the highest surface-to-mass ratio would make it to the top of the vertical tub

That's probably why they never show it on sand - it would suck it up. Things with the highest surface-to-mass ratio would make it to the top of the vertical tubes — things like dust, cigarette butts, and candy wrappers. Pebbles have too low of a surface-to-mass ratio to get sucked all the way up the vertical tube.