When there is a cloud with positive and negative electron and another without negative electron are stood next to each other. 
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During the thunderstom, the thunderclouds are charged due to the friction between water molecules in the rain clouds and the fast moving air molecules. Then, the bottom of the cloud become negatively charged and the top of the cloud become positively charged. Lightnimg occurs when the electrons at the bottom of the clouds move to the positively charged area of another cloud OR the earth's surface because the earth's surface is always positively charged..
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