In assessing negligence, what is the last clear chance rule?

"Contributory Negligence" is the legal defense equivalent of, "Yeah, but they were stupid, too!" Imagine Bob negligently pouring oil out on a sidewalk. Later, Sam comes running by while holding scissors and slips on the oil, landing on his collection of now-bloody scissors. Sam would argue that Bob is liable for his injuries because he negligently caused the sidewalk to be slippery. In response, Bob may say, "Sure, I did that -- but Sam is the fool who was running with scissors. Sams negligence contributed to his injuries.

In response, Sam may claim that Bob saw him running towards him and knew the sidewalk was slippery, and that Bob had the last clear chance to warn Sam of the slippery sidewalk, but he did not. In other words, Sam argues that even though he was also negligent, ultimately Bob was in control of avoiding the accident had he acted and, because he did not act, the accident occurred.