Ordinarily, a plaintiff in a personal injury action has the burden of proving that a defendant's negligence caused his or her injury. However, when a plaintiff proves that two defendants have committed negligent acts, and it is impossible to determine which act caused the plaintiff's injury, the burden of proof shifts to the defendants. Each defendant has the burden of proving that his or her negligent act did not cause the plaintiff's injury.

For example, defendant A, defendant B, and a plaintiff are hunting. Both defendants fire their rifles in the plaintiff's direction. One bullet hits the plaintiff. The plaintiff files a personal injury action against both defendants. At trial, the plaintiff proves that both defendants were negligent in firing their rifles in the plaintiff's direction, but it is impossible to determine whether the bullet that hit the plaintiff came from the rifle of defendant A or defendant B. Therefore, each defendant has the burden of proving that the bullet did not come from his or her rifle. If neither defendant can prove this, then both defendants are liable for the plaintiff's injury.

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