Sally, email asked:
What is a mirage?
A mirage is a trick of light seen in hot places like deserts (hence the association with thirsty travellers) and road surfaces. Like the end of a rainbow (another light trick) you can never actually reach a mirage. They form because when air is heated up light rays pass through it more rapidly, causing them to bend, or refract, upwards. They bend upwards because warm air close to the road surface, for instance, allows the light to pass through more rapidly than cooler air higher up, but when the rays reach your eye they are interpreted as coming from the road surface (like a reflection), rather than the sky, and so you see a mirage.