When turned on and stood on the table, this electric toothbrush moved left in a more or less straight line. Here's my theory as to why. The brush head twists backwards and forwards - not whole turns but a bit clockwise and a bit anti-clockwise. The head is not on the axis of the brush so it creates a torque. There is a reaction torque, equal and opposite as the friction on the base pushes back. This nudges the base around a little bit. Done alternately, this seems to add up to a linear motion, a little like shuffling.
