If the heart is made of muscle, how come it doesn't need to rest like other muscles? The heart is made of a unique kind of muscle, called myocardium. All muscles contain mitocondria, which have enzymes to convert food into energy. In most muscles mitocondria make up only one to two per cent of the volume of the muscle. This means that muscles get tired and need a rest when the mitocondria are unable to keep up with increased energy demands. In the heart, however, these mini-powerpacks make up as much as 35 per cent of its volume. This is good news, because it means your heart never gets tired. Why do humans need to eat fruits containing Vitamin C but dogs and cats don't? Vitamin C, or ascorbic acid, is found in oranges, lemons, tomatoes and many fresh vegetables. Among other things, the body needs Vitamin C in order to produce collagen, which is a group of proteins found in skin, bone, tendons and cartilage. A severe lack of Vitamin C results in scurvy, causing anaemia, bleeding gums and bleeding under the skin. These days scurvy is relatively rare. However it was common among sailors on long sea voyages where rations often consisted of salted meat, hard biscuits and rum. In 1747 Scottish naval surgeon James Lind cured scurvy by making sufferers eat oranges and lemons. Eventually British sailors were required to drink lime juice to protect them against scurvy. Because of this, British sailors were called 'limeys' by Americans. While humans need to eat food containing Vitamin C, most animals make their own. So we don't need to give our pets Vitamin C in their diet. Why is the sky blue? Because it matches the sea and nature likes symmetry. Actually, during clear days, when there's not too much pollution around, the sky appears to be blue as a result of the scattering of light rays by tiny particles of dust in the atmosphere. Rays of light with longer wavelengths - red and yellow - are not scattered too much while those with shorter wavelengths - at the blue end of the spectrum - are thrown about all over the place. The result is that the sky appears blue. If there is a lot of dust in the atmosphere, other rays, besides blue, get scattered, so the sky looks white or hazy. Why is a pirate flag called the Jolly Roger? According to David Mitchell in his book Pirates, Jolly Roger is a derivation from Old Roger, a synonym for the devil. Another explanation has an Indian origin - English pirates in the Indian Ocean began to refer to the red flag of the Tamil pirate Ali Rajby by his name and 'Ally Roger' or 'Olly Roger' was later corrupted to Jolly Roger. Yet another simpler possibility is the origin of the word Roger, which in English slang, means a vagabond rogue.