Here's a joke I made up in the fifth grade.
A general was preparing for a tank battle and needed more men to sit up top with an opened hatch manning a machine gun. They called that position the "hatches." All the hatches were killed in the prior battle and so he asked the seargent if he thought they could fill all the empty slots with volunteers. The seargent guaranteed the general that the troops under his command were all brave and patriotic and there would be no trouble getting volunteers. When it came time for the battle the general asked where all the hatches were. The seargent told him that no one volunteered and that the ones he'd ordered to come had all run away. The general was furious and said to the seargent, "Don't count your hatches before they chicken!"