The word contract has two main meanings:

1. An agreement between people or groups that explains what each side promises to do. When your parents sign a contract to buy a house, the contract spells out exactly how much they'll pay and when the previous owners must move out. Professional athletes sign contracts that state their salary and how many years they'll play for a team. A contract makes an agreement official and binding, meaning both sides are expected to keep their promises. Breaking a contract often has serious consequences, which is why people read them carefully before signing. Even kids make informal contracts: when you agree to walk your neighbor's dog for a month in exchange for twenty dollars, you've made a simple contract, even if you didn't write it down.
2. To become smaller or shorter. Muscles contract when they tighten and pull, which is how your body moves. When you bend your arm, your bicep muscle contracts. Metal contracts when it gets cold, which is why bridges have special joints to handle expansion and contraction from temperature changes. A teacher might say that the words “do not” contract to form “don't.” Pupils in your eyes contract in bright light to protect your vision. This meaning is essentially the opposite of expand.