If you are caught outdoors in a thunderstorm, why shouldn't you stand under a tree? Can you think of a rea - son why you should not stand with your legs far apart? Or why lying down can be dangerous? (Hint: Consider the electric potential difference.)

Because its dangerous.

Explanation:

During lighting strikes, there is discharge or energy transfer (electrons) from the clouds to the earth, these electrons flow through the path with least resistance between the cloud and the earth. Also the electric field around the tip of the leaves are strong, which makes trees a great target.

we as humans have lower resistance than trees, that is to say, the lighting may leave the tree and flow trough the body to the earth.

The tree and the ground around it are then raised to a high potential relative to the ground some distance away.

If you stand with your legs far apart, one leg on a higher-potential part of the ground than the other, or if you lie down with a potential difference between your head and your feet, you may find yourself a conducting path.

If it is also raining, the electricity may transfer down the wet tree to the wet ground and shock anyone standing near the tree.