The Science Museum of Minnesota had a little plastic connector that allowed you to fasten two 2 liter soda bottles together. I don’t think the threads of a regular PVC pipe connector would do it, but similar. Extremely simple. Might be available somewhere on the internet. You fill one bottle 3/4 full, screw them together with this connection, flip over and spin the upper bottle in a cone like motion. As the water drained through the connector into the lower bottle, the spin imparted with the swirl got the water going into a tornado likeness.
My kids did this on and off for years whenever they’d find the contraption and then put it away for a few months. The 5th grade teacher had a couple we gave them and I’d see them in the classroom so I assume the kids were playing with them and getting a little dose of science.
https://www.youtube.com/watch?v=0LfZFGcGc_I
Dale