How thunder occurs?

The thunder occurs of coincidence of two clouds the thunder occurs.

clouds are very light in weight then how the thunder creates. I think thunder is nothing but a kind of energy like electricity. so How it happens. can you explain scientifically

when positively charged clouds merged with negatively charged clouds a spark produces which leads to thunder.

what you mean by positively charged and negatively charged

Actually during thundering two opposite charged clouds combines which causes noise like blast or like horrible .

Isha has correctly explained the reason behind thunder. Nice explanation Isha.

Can you tell me isha how clouds get charged that to oppositely charged