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Last Word is Â鶹´«Ã½â€™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

Hominin fossils and artefacts are often found in metres-deep sediment in caves. What caused the mess? Didn’t hominins clean?

Allen Reynolds,

Greenfield, New Zealand

If I were able to create two negatively charged black holes, could they repel each other? Or does gravity always win?

Michael Diesso,

Princeton, New Jersey, USA

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

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