When a tree is struck by lightning, the liquids inside the trunk and bark turn to gas instantly, leading to high pressure and literally explosion of anything that is between the gas and the open air. Usually, the lightning current runs just underneath the bark, down to ground, and the tree is scarred by a strip of blown-away bark. It usually survives such a strike. Sometimes, the current may run down near the center of the trunk, and then there may be little left of the tree afterwards. This is one of the reasons why it isn't safe under a tree during lightning - the exploding timber will blow away at high velocity and act like projectiles. It is also not safe to be under a tree during lightning because the high current from lightning will travel along ground radially away from the tree and lethally shock you. Also, lightning may jump from the tree being struck, to the person taking shelther there.
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