Hello There! ^_^
Your question: What were the three main Indian groups before European explorers arrived..?
Your answer: The Mayan, the Aztecs and the Incas were the three main Indian groups before European explorers arrived.
Hope this helps!
1. gravity keeps the water moving downward or just in general
2. the water has to move across the surface for so long till eventually it just erodes away.
those are the only two i know sorry.
Lighter, positively charged particles form at the top of the cloud. Heavier, negatively charged particles sink to the bottom of the cloud. When the positive and negative charges grow large enough, a giant spark - lightning - occurs between the two charges within the cloud. Hope I helped you out :)
