When I got home from school today I found a plastic bottle, a balloon and a nail.
I poked the nail into the bottle near the bottom to make a hole. Then I put a balloon inside the bottle and folded the end of the balloon over the top of the bottle.
Then I blew the balloon up inside the bottle, put my thumb over the nail hole, and filled the balloon up with water.
Then I handed it over to my dad, and as my thumb left the nail hole he got squirted with water
You can also do this with a screwtop bottle cap. If you put the cap on you can take your thumb away and you don't have to worry about it squirting you by mistake.