This is a fun magic trick that can be mastered very quickly.  The Magic Water Bottle has two chambers on the inside.  When you cover the hidden vent hole on the top you can empty the small chamber through the spout.  When you remove your finger from the secret spot, the water from the second chamber refills the first one. You can repeat the trick 8 to 10 times and convince your audience you have a magic water bottle that refills itself.