Imagine an HIV test that is 95% accurate (false positive rate of 5%) and around 2% of the tested population is infected with HIV. What is the probability that you actually have HIV when your test comes back positive?
You’re standing 11 feet away from a doorway and with each step you move halfway to the doorway. How many steps will it take for you to get to the doorway?
Technically, the answer is infinity. If you keep going halfway, you’ll never actually make it to the doorway.
But in practice, after 10 steps you’ll be 1/8th of an inch to the doorway and anyone behind you would say to stop being a baby and take a real step through the doorway already.
Milton was out for a walk when it started to rain. He did not have an umbrella and wasn’t wearing a hat. His clothes were soaked, yet not a single hair on his head got wet. How did this happen?
Milton sadly suffers from alopecia, more commonly known as baldness. The old coot was as bald as a ping pong ball, leaving not a single hair on his head to get wet.
Finley’s friends all chipped in to buy her a wedding gift. At first, ten friends agreed to contribute the same amount. Then two “friends” dropped out. The remaining eight had to add another dollar each to cover the cost of the gift. How much did the gift cost?
A group of ten people are going out for pizza but only two of them have an umbrella to keep them dry. Somehow they are able to walk all the way to the pizza place without getting wet. How?
A variation of this brain teaser includes laser printers (they were invented by Gary Starkweather) and bulletproof vests (invented by Casimir Zeglen using silk, though modern vests use Kevlar, so it’s halfway right).