Jim Al-Khalili talks to neuroscientist and occasional stand up comedian, Professor Sophie Scott about how she is using brain imaging techniques to reveal secrets of the complexity of brain activity when we speak and when we hear others speak. And Sophie Scott explains why laughter is such an important human social tool. But why is it that if we're laughing hard it can completely override our ability to speak? Also why it's not just humans who have a funny bone: even rats laugh.