Why do we laugh when tickled?

In this question and answer special, the Naked Scientists get stuck into your queries, like why are planets round? Why do we laugh when tickled? Does wearing glasses make your eyesight worse? And how many trees could offset carbon emissions? Plus, in the news, the app that could help you get over jet lag and the Heartbleed bug which could affect your internet security. Like this podcast? Please help us by supporting the Naked Scientists