You will need eight matchsticks for this teaser.

Take six matchsticks and arrange them into a 2x1 rectangle. Next, take another matchstick and add it to one of the long sides of the matchsticks. Finally, take the last matchstick and add it to the other long side, in the same direction as the last matchstick you have laid. You should now have what looks like a glass of water.

Now, how can you increase the volume of the water by moving only two matchsticks?

You cannot:
Move the uppermost horizontal matchstick.
Submerge the matchsticks in the water.
Bend, break, or stack the matchsticks.