Well kiddo, there was a man named Euhemerus who lived a loooong time ago in ancient Greece. He came up with the idea that the stories and myths that people told about gods and goddesses might actually be based on real people who lived a loooong time before, and over time people started to see them as gods.
So let's say there was a very brave and strong man named Hercules who did amazing things, like slaying a lion and cleaning out some really gross stables (ew!). People would tell stories about him and over time, they might start to believe that he had special powers and was actually a god.
Euhemerus called this theory "euhemerism" (wow, that's a fancy word!). Basically, it means that the myths and legends that people told about gods and goddesses were actually based on real people who did incredible things, but over time their stories got mixed up with other stories and people started to see them as more than human.
So, next time you hear a story about a god or goddess, just remember that they might have once been a real person, just like you and me!