The Best Rule of Thumb is a Thumb Rule

This reddit thread asks "What can a person learn in 10 minutes that will be useful for life?"

The top-voted answer: "Use your hand span — thumb to pinky — as a built-in measuring tool. Once you measure it you will never forget it."

Even better: If you happen on a person who's unconscious, lift their legs in the air as you call for help, and keep them elevated until help arrives or the person comes to.

This shunts about a third of their circulating blood volume back to the heart and may prevent imminent cardiac arrest.