• Wet, wet, wet: It is the class of space rocks that includes Itokawa that may have brought half as much water as is in Earth’s oceans (11 May, p 18).
• Mind like a sieve: The reason that Harald Helfgott’s prime number-finding algorithm is purelytheoretical is that he hasn’t optimised it (11 May, p 11).
• Quite crowded: There are 10 million bacteria per square centimetre of human skin, on average (13 April, p 28).
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