Police cars are fast! They have special engines that make them zoom really quickly. When someone needs help, like if they feel scared or need assistance, police cars hurry to get there.
Imagine if you see a toy rolling super fast! That's how police cars move through the streets. They want to make sure everyone is safe, just like when you call for a parent when you need help.
So when you hear a siren, it means a police car is on its way fast to help someone who needs it!