Fill one bottle until it is 2/3 full. You can also add several drops of food coloring.
Dry the tops of both bottles, invert the empty bottle onto the full one, and firmly wrap the tape around the two bottlenecks. Make sure the bottlenecks are tightly taped so that no water can leak through.
When the device is ready, invert the bottles so that the full bottle is on top. Swirl the device and watch the water as it spins against the sides. Perform the experiment above the sink, just in case.
If you don’t have any duct tape, fill a clear bottle with water until it is 2/3 full, then turn it upside down and swirl it until you see a whirlpool forming.
What’s the science behind this? When you empty a bottle or any other container of water, air must flow into that space. Otherwise, air pressure will stop the water from flowing down. When you fill a bottle with water, the air inside the bottle must escape so that water can flow in.
When you empty a bottle, air rises through it in the form of bubbles. If you place the full bottle over the empty one and don’t move them, you’ll see air bubbles going up. As they go up, the bubbles slow down the flow of water.
When a whirlpool is created, the water moves in a circular motion against the bottle and then down. A hole is formed in the middle, which allows the water to move upward. As the water flows down faster, the centrifugal force created by the swirling motion pushes the water against the bottle and the hole in the middle grows. As the hole grows, the water has less room to flow downward, and it slows down. In this way, a balance is achieved and the flow is stable.
Watch the water as it spins inside the empty bottle – and against its sides. The water’s circular motion continues as it flows down into the empty bottle and disperses.