As a gamer, reading “What half-life measures in nuclear physics” and not thinking about Valve’s industry-changing shooter feels like sacrilege. That said, knowing Half-Life will help a lot when you’re trying to solve this clue.

I've already had fun with this clue and we're just getting started. Here's the answer and the reason for it.

Eight on the other hand is “DECAY” because outside of the video game, in nuclear physics, a half-life is the time it takes for a radioactive isotope to lose half of its radioactivity – in other words, for its radioactivity to decay.

Without Valve's Half-Life or its fantastical universe, this is the easiest five out of five for difficulty I've ever given – and I'm sticking with a five out of five anyway. Unless science was one of your strong points growing up (it was my worst subject) and you have a super-awesome memory, this is a trial tip. Also, playing Half-Life doesn't guarantee you'll get this right – it's an absolutely brutal tip, and taking on a real-life Vortigaunt would be easier.