large container of water, like a bathtub or a fish tank
two see-through plastic cups
How to do it
Take one of the plastic cups, turn it upside down and push it into the container of water. This cup is now full of air.
Turn the second cup upside down and push it into the container of water. Turn the cup to the side and fill it with water.
Put the cups close together and pour the air into the cup filled with water. It's like you're pouring air instead of water.
This works because the air in the first cup is a giant bubble. A bubble is a hole in water. This bubble stays inside the cup when you put it upside down into the water. Then, when you tip the cup, the bubble moves through the water, up into the second cup. Air bubbles always move up in water. That's because air bubbles float.
Try pouring the bubble out of the cup and catching it with the strainer. The bubble is too big to get through this strainer. The bubble would need to be the size of the holes in the strainer to get through. Try making the bubble the right size by shaking the strainer.