Instructions: Press and hold the left mouse button on the blue square. Let go when the shot's size gets as big as you want it. If you need more zombies for a new try, press the button to the left of the blue field.
<a href="http://www.funpuzzlesgames.eu/index.php?og=Zombie+Catapult%3E%3E%3E%3E4769b66faa01314e&ogc=Puzzles" target="_blank">Zombie Catapult</a>