During a thunder storm, if a lightning hits the ground near a cow, then the current starts to spread out in the groud. Since the ground is wet, it is easier to spread horizontally, and also, the ground is somewhat positively charged. (See Chapter 11. How does lightning start and end?)

When the current starts to spread when a cow is eating grass, the current flows from the cow's mouth (A), goes through its leg (B), and goes back to the ground. The current will flow until the voltage becomes zero.

Because cow's hind leg (D) has lower potential energy than its front leg (C) , the current can flow through the cow. The cow becomes a part of electric circuit.

Then the cow gets a big electric shock and it usually dies.