Don't you pay attention in school? Well you know how when you rub your feet on carpet and then touch something, it occasionally will shock them? Well that's what happens with clouds. Static electricity!!!!! Hope this helped!
lightning is still technically considered a natural phenomenon due to the fact that we don't understand the mathematics involved in tabulating the exchange in nuclear particles between particles but there are generally accepted theories about the event that u can choose to accept.
A lightning appears when the particles of the clouds clash against each other and extract electricity... Then the cloud has a positive and the ground a negative charge and to neutralize the charges the electricity breaks out and goes the shortest way to the ground^^ I'm in 8th grade