Why Do Some People Always Have the 'Last Vote'?

Imagine you and your friends are deciding what to eat for dinner. If everyone says pizza, but you really hate olives and say 'no', the whole group has to think again or find a new place. In the US government, the president has this special "no" button called a veto. When Congress passes a law that the president does not like, they can hit that button to send the law back. It is like saying "not yet". Congress isn't completely defeated though. If enough people in Congress vote again and agree strongly enough, usually two thirds of them, they can override the veto and make the law happen anyway. This back and forth helps keep power balanced so no single group gets too bossy.