Handcuffs are things that the police use to stop someone from moving their hands. They can be like bracelets made of metal or plastic that they put around a person's wrists. The police use them when someone is being arrested and they need to make sure that the person doesn't run away or hurt anyone. The handcuffs are held together by a special lock that only the police have the key to. The person who is wearing the handcuffs can't get out of them until the police decide to take them off. It's really important to only use handcuffs when it's necessary and to be very careful when putting them on because they can be uncomfortable and even hurt the person wearing them.