Am I a snake: my yawns spray saliva!

This week, journey times to Mars, how stars come by the matter inside them, why some people feel compelled to eat paper, how coronavirus antibody levels correlate with Covid-19 severity, whether water really goes down the drain in different directions on opposite sides of the Earth, and why a yawn produces a spray of saliva. Join Dr Chris Smith and Azania Mosaka for the answers.