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Last Word is ¿ìè¶ÌÊÓƵ’s long-running series in which readers give scientific answers to each other’s questions, ranging from the minutiae of everyday life to absurd astronomical hypotheticals. To answer a question or ask a new one, email lastword@newscientist.com

I use a petrol lawn mower – does it emit more carbon than the grass captures?

Hugh Meteyard,

Theydon Bois, Essex, UK

Heat a ceramic mug of water in the microwave, tap it with a steel spoon, and the pitch is higher than before it went in. Why?

Francis Westfield,

Stoke-on-Trent, Staffordshire, UK

To answer this question – or ask a new one – email lastword@newscientist.com.

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