Cold case

Okay kiddo, you know how sometimes when you lose something like a toy or a book and you look for it everywhere but you can't find it? Well, sometimes the police have a really hard case to solve because it happened a really long time ago and they don't have any clues to help them figure out what happened. They call this a "cold case" because it's like the case got too cold to solve. It's like someone hid your toy really well and you can't find it no matter where you look. So the police have to do a lot of extra work and use special skills to try to find new clues and solve the cold case. It's like they have to go on a really hard treasure hunt to find the missing pieces to solve the mystery.