Can frozen onions make you cry?

Why do onions make you cry and will a frozen onion have the same tear-jerking effect? What medicinal qualities does aloe vera possess? Is ice-crunching harmful? What's the best way to prop up metabolism as we age? What can I do about stress incontinence? Plus, news of what Tweets posted online can tell us about the population's collective mood, and a new way to tell who needs extra investigations after a heart attack... Like this podcast? Please help us by supporting the Naked Scientists