I recently received a free SpillNot through a promotion at Arbor Scientific. This really is a cool tool!  With this device, you can carry a beverage or even swing it around in circles without spilling!  It’s a great illustration of the effects of horizontal vs perpendicular acceleration on a cup of liquid and the physics of circular motion. Click on the link to see a video of the SpillNot in action and read an explanation of the physics behind it.  My children were fascinated with this cool tool, though I have to admit that we did have a spill!