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Question every parent dreads: why is sky blue?

Parents would rather their children asked them where babies come from than why the sky is blue, a poll suggests.

The survey reveals many parents dread being asked science-based questions, with nearly one in five hardly ever talking to their offspring about the subject.

Just a third regularly talk to their children about how science works, explaining ideas such as why steam comes out of a boiling kettle.

And the poll, which was commissioned by Shell Education Service and hosted on the Mumsnet website, reveals that it's the science questions that leave parents stumped.

Almost one in three said they would not know how to answer if their child asked "Why is the sky blue?".

More than one in five said they would struggle with "Why does the car work?"

The third most difficult question is "Why can birds fly?" chosen by 15pc of parents, followed by "What is water made of?" (10pc) and "How do fish breathe?" (9pc).

Just 1pc of parents said they would be stumped if asked "Where do babies come from?" the poll found.

Almost all (99pc) parents said it would help their child's progress in science if they talked to them about the subject at home.

One in 10 of the parents said they did not talk to their child about science because they did not feel qualified to do so.

Shell Education Service is publishing a booklet containing easy experiments for children and parents to try at home.

Rachel Foster, associate editor of Mumsnet, said: "Some parents worry about giving the wrong answer to some of the more difficult science questions asked by their children, and as a result are avoiding discussing the subject altogether.

"Mumsnetters are always looking for fun and educational things to do with the family, particularly over the long summer holiday, and this new booklet from Shell presents a great opportunity for families to learn together while also having fun."

So why is the sky blue? Simple -- as light moves through the atmosphere most of the longer wavelengths, like red, pass straight through.

But blue light, which has a short wavelength, hits the air molecules and is scattered through the sky.

This "Rayleigh scattering" means blue light reaches the observer from everywhere he looks -- so the sky looks blue.