This simple device is modeled on the idea of the “Greek Waiter's Tray” or “SpillNot”. You tie a strong string to the loop at the top and then place a cup of water on the tray. As long as you do not move too abruptly the water will not spill. With just a little practice you can swing this in a large circle so that the cup goes upside down.

The motion of the cup and liquid is tangent to the path the tray takes and the cup experiences a net force towards the center of the circle (a centripetal force). Your students will likely be sure the cup is glued to the tray.