Why will I be moved backwards and forwards in the MTR when the train stops? LEE CHI-TIM Whenever the MTR starts or stops, we are thrown backwards or forwards. This is due to inertia, the resistance of an object to any change in its state of motion. When an object is at rest, it is reluctant to move (think of trying to get up in the morning to go to school and you'll know what I mean). When a thing is moving, it is reluctant to stop. This was first recognised by that clever chap Sir Isaac Newton in his laws of motion. Sir Isaac also discovered gravity, which was lucky because otherwise we'd all fall off the earth and spin off into outer space. Newton's first law of motion states that every body continues in a state of rest or uniform motion in a straight line unless it is acted upon by an external force. When the MTR stops, the inclination of our bodies is to continue moving, so we end up lurching forward. Similarly when the train starts, we would rather remain stationary and we get jolted around. Why is there so much noise if you put your head underwater, even in the bath? If we are underwater, we can often hear things which we cannot hear above water. This is because sound travels better and faster through liquids than gases. In air at zero degrees Celsius, sound travels at 332 metres per second. At 20 degrees, sound travels faster - at 344 metres per second. For every degree rise in temperature, sound increases its speed by about 0.6 metres per second. In liquids, sound travels four times faster than in air and is conducted better, which is why we can hear better underwater. In solids, sound travels even faster. The more solid the matter, the faster and better sound is conducted. Also, underwater you receive sound vibrations directly into your skull, rather than just through your ear, which is why underwater noises also sound rather weird. Whales take advantage of the excellent sound carrying characterstics of water to communicate with each other. Whales can carry on a conversation across thousands of kilometres of ocean. However, the increase in the number of power-driven ships means whales are often unable to hear each other due to the noise of ships' engines. What kind of animal is the horseshoe crab? Is it really a crab? Despite its name, the horseshoe crab, scientifically known as tachypleus tridentatus, is not a crab, but a relative of spiders and scorpions that lives in shallow sea water. Horseshoe crabs are 'living fossils' - members of a group that has changed little in millions of years. They are thought to have evolved very slowly because they eat a wide range of foods and live in a stable habitat. They are now being studied for research into sight and their blood is used in drug research.