Light is supposed to be massless so how is it that gravity makes a beam bend? Plus, we ask what the hand wash adverts mean when they claim to kill 99.9% of known germs. Like this podcast? Please help us by <a href="https://www.thenakedscientists.com/donate">supporting the Naked Scientists</a>

What does it mean when soaps claim to kill 99.99% of germs?

Each week we set out to solve one of the world's weirdest, wackiest, funniest and funkiest scientific puzzles. And along with the answer there's a brand new question to think about for next time...