Darwin was a very smart man who came up with an idea that was both exciting and a little scary. He noticed that animals and plants are all different in different places. He wondered why that was.
So he did a lot of studying and watching and thinking. He came up with an idea that explained why animals and plants are different. It's called "natural selection."
Imagine you have a lot of different toys to play with, but you can only keep your favorites. Which toys you keep will depend on what you like to play with the most. This is kind of like what happens with animals and plants. Some animals and plants have traits that help them survive in their environment, and those are the ones that get to keep living and reproducing.
This means that over time, certain traits become more common in a certain group of animals or plants. For example, if there's a lot of predators around, the animals that are good at hiding or running away will be more likely to survive and have babies. Over time, more and more animals in that group will have those good hiding or running skills.
This idea was dangerous because it challenged a lot of people's beliefs about how the world worked. Some people believed that all living things were created the way they are and couldn't change. Darwin's idea said that living things could change and develop over time.
But it was also exciting because it helped explain how living things evolved to be the way they are today. It helped scientists understand how one kind of living thing could eventually turn into a completely different kind of living thing over millions of years.