When you flip the connected bottles and swirl them, a spinning funnel of water forms as the liquid drains from the top bottle to the bottom. That funnel shape is a vortex — a spinning flow of water you can watch in action. Adding food coloring or plastic confetti makes the swirling motion easier to see.

A tornado is a spinning column of air — a vortex — that pulls into a funnel shape as it reaches from cloud to ground. When you swirl two connected bottles, water drains in that same spinning motion. Add food coloring or plastic confetti and the vortex becomes visible, letting you study how the spinning funnel behaves as it forms.