Okay kiddo, so let's imagine you have a toy car that's red but you want it to be blue. What can you do? You can substitute the red color with blue color by painting it blue.
Similarly, in chemistry, there are some molecules that have different parts called functional groups. These functional groups can be changed or substituted by other groups to create new molecules using what's called a substitution reaction.
Let's say you have a molecule that has a functional group of a hydroxyl (-OH) group, and you want to replace it with a different functional group, like a chlorine (-Cl) group. You can use a reaction called a substitution reaction to do this.
In this reaction, the hydroxyl group is replaced with the chlorine group. It's kind of like trading one piece of your toy car for another, but in this case, it's happening between different parts of the molecule.
Substitution reactions can be used to create all sorts of new molecules! And just like changing the color of your toy car, these reactions can lead to new and exciting things in chemistry.