The simple one-word answer is melanin, the pigment that gives skin and hair their colour. Darker skin has more of it, giving protection from the sun's ultraviolet rays, which can cause skin cancer.

You can explain that, in general, people with ancestors from tropical regions have darker skins than those who lived in cooler climates. “Originally this variation was down to where you lived in relation to the Equator, which influences the level of sun exposure,” says Dr Toomas Kivisild, a lecturer in human evolutionary genetics at Cambridge University.

Some scientists believe that lighter skin offered stronger chances of survival to the people who first migrated out of Africa thousands of years ago because it absorbs higher levels of bone-strengthening vitamin D from the sun. “The alternate view, which Charles Darwin proposed, is that different skin colour is down to sexual selection, the idea being that our European ancestors preferred to mate with those who had lighter skin, but there's no good support for this,” Kivisild says. At which point there's no escaping that such a question throws up sensitive cultural issues associated with potential prejudice. So although factual answers are key, it is also important to encourage your child to accept the range in skin colours as no more than normal variation.

It could also be worth asking Alice why she's asking; has it come up in class or in the playground? Then use the question as an opportunity to talk about your family values. “Discuss what you believe in as a family, about being respectful and viewing people equally,” says Claire Halsey, a child psychologist and parenting expert.

Ask Your Father: why haven't I got a brother or sister?

Alfie, 9, wants to know why he's an only child when his friends have siblings

Background

• How come Daddy can't work the washing machine?

• Ask your father: why aren't you married to daddy?

• Ask your father: why are you on a diet?

• Ask Your Father: What's the biggest number in the world?

• Ask your father: Mummy, why can't I live forever?

Children will be aware of your reaction and how it reveals your anxieties and doubts. Halsey continues: “Embrace this as a curiosity question; see it as part of their development, rather than getting nervous about what it may lead to. It's about emphasising that we're all different in various ways, from our eye and hair colour to our height and shape, and stressing that each difference brings a richness to our society. These beliefs need to be spoken about, rather than assumed.”

EMMA COOK

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