Cut the plastic tub down, so that it is only 2 to 3 inches high. Drill a small hole in the center that is just large enough for your bolt to pass through.
Cut out a disk-shaped piece of the plywood just large enough to fit inside the tub. Drill a hole through the center for the bolt.
Use the duct tape to attach the fan on one side of the wood disk. The fan should be facing at an angle 45 degrees from a line drawn from the center of the disk to the edge.
Tape the level on the opposite side of the disk, so that the glass bubble runs along a line drawn from the center of the disk to the edge.
Roll the marbles or ball bearings around in a teaspoon of oil in a bowl. Then pour them into the plastic tub.
Place the disk inside the tub, so that it rests on the marbles or bearings. Pass the bolt through the hole in the disk and the hole at the bottom of the tub, placing a washer at each end. Tighten the nut on the bolt, but leave it a bit loose.
Place your demonstration on your desk, and have your students gather round. Turn the fan onto its lowest speed. As the disk starts to turn, have the students note how the bubble in the level has moved off center.
Turn the fan to the higher speed, and have the students note how the higher speed change makes the bubble move even farther off center. Discuss the results.