Why can't you tickle yourself?

This week, why does a bout of flu leave you with blocked ears? Can grapefruits be engineered to taste like meat? Why is genetic diversity highest in Africa? Why can't we tickle ourselves? And why does seeing someone else suffer make us experience pain ourselves? Plus, news of a treatment for chronic fatigue syndrome, why antivirals might abate Alzheimer's and how seaweeds make corals seasick...