Okay, imagine you have three toys: a car, a ball, and a doll. Now let's say your friends came over to play with you, and they also brought three toys: a train, a kite, and a teddy bear.
You all decided to play outside and have a race with the toys. But before the race, you wanted to show off your car toy to your friends. You put the car on the ground and said, "Look, this is my car toy!"
Your friends were curious about your car toy and asked if they could try it too. You said it was okay, so your first friend picked up the car and played with it for a bit. Then your second friend did the same thing.
That's what a common base event is: a situation where different people or things use the same starting point or foundation. In this case, the starting point was your car toy, and your friends all built from there by playing with it.
It's like a tree trunk that has many branches coming out of it. The tree trunk is the common base event, and the branches are all the different things that stem from it.
I hope that helps you understand what a common base event is!