Let's try a little gedanken (thought experiment)....
Anyone who thinks sand would make a good thing to have within 100 yards of a FIRST competition, should imagine sprinkling just a little over their robot (on the Victors, transmissions/gears, bearings, conveyor belts, treads, etc.) (maybe get a little into the driver's controls too) and then imagine trying to use the robot.
Now, tell me if you have changed your mind. :-)
Nothing smaller than a ping-pong ball inside or outside of containers please.... This includes water, even if it distilled and consequently is non-conducting.
Even something as big as ping-pong balls is asking for lots of trouble if they escape containment while above a robot(s).
Blake
PS: If you remain unconvinced, get permission to destroy a robot, and then try the sand experiment. :-) :-)