Make sure you ask an adult to help you use the stove.
Heat up the water in a teapot. It doesn't need to be boiling, but it does need to be hotter than tap water.
Take off the screw top from the glass bottle and make a hole in the top with a pair of scissors. You may need an adult to help you make the hole.
Fill the bottle halfway with the cold water, and add a few drops of food coloring to the cold water so it's easier to see. Screw the top on as tightly as possible.
Push the straw through the hole so that it is in the water.
Press clay around the straw to seal up the hole.
Put a plug of clay in the end of the straw like a cork.
Pour water from the teapot into a large bowl. Put the bottle into the bowl. Pour more hot water on the bottle. Leave it for a few minutes. Watch what happens.
The reason this happens is because the hot water in the bowl warms the air in the bottle. As the air gets warm, it expands and takes up more room in the bottle. Then, it pushes the water up the straw, out of the bottle, into a spray that looks like a fountain.