Okay kiddo, so a long long time ago there were only regular bikes with thin tires that people rode on paved roads. But some people wanted to explore and have fun riding on rough and bumpy dirt paths and rocky terrain. So some really smart people started tinkering around with regular bikes and inventing ways to make them strong and sturdy enough to handle the rough terrain.
They added big fat tires that could grip the ground better and special suspension to absorb shocks and bumps. They also made the frame of the bike stronger and added gears to help riders go up and down hills easier.
And then, these tough new bikes became known as "mountain bikes" because people started taking them to the mountains and riding on rugged trails and steep slopes. People were so excited about these new bikes that they started racing on them and having competitions to see who could ride the fastest and go the farthest.
And that's how mountain biking was born! Now mountain biking has become a really popular sport and people all over the world ride these amazing bikes on crazy trails and have a lot of fun doing it.