Injective function

An injective function is like a special type of machine that takes in things and gives back different things, but it's really important that it never gives the same thing back to different things that it took in. For example, if we have a machine that turns apples into juice, we need to make sure that if we put in two different apples, we get two different juices back. We wouldn't want to get the same juice for both apples, because that would be no fun! So an injective function makes sure that everything it puts out is unique and special, just like we are!