A The Earth's core is solid because of "pressure freezing" - a process in which the pressure of the surrounding mass of material forces something that would otherwise be liquid to become solid. The first person to confirm that the Earth had a solid core was Inge Lehmann, who died in 1993.
QWhen a budgie in a cage starts flying, what happens to the total weight of cage and bird?
A The total weight, as measured by a weighing scale under the cage, would go down. When the budgie is on its perch, the total weight is a combination of the weight of the cage and the weight of the bird. When the bird launches itself into space, its weight no longer has any effect on the scale the cage is on. However, if the bird were to be in an airtight container (don't try this at home), the weight would remain the same. This is because as the budgie beats its wings in flight, each stroke pushes down a column of air with the same weight as the bird.
QWhat makes the white trails which are left behind when a plane has passed overhead?
A One reason is that water vapour from the plane's exhaust freezes in the cold air to make ice particles. Also, as air passes over the plane's wings and tail, its pressure drops, causing it to cool rapidly, which condenses any water vapour. Concorde also creates strange air and water patterns on take-off and landing: a mist often forms over the top of the wings as the lower-pressure air just above them is less able to hold water than air underneath. This creates a mist.
QWhy doesn't the chocolate in chocolate chip cookies melt when they're being cooked?
A To all intents and purposes, it does, but it is held in place by the dough around it. However, the chocolate doesn't melt as much as ordinary chocolate would, because it has been tempered. Tempering is the process where chocolate is repeatedly heated and cooled until a crystal-like structure appears within it. This makes the chocolate more stable and gives it a glossy shine and a crispy surface.
QHow can birds stand on high voltage electricity wires? Don't their two feet make a circuit?
A Yes, birds' feet do make a circuit that passes through their bodies. But as with any circuit, the electricity prefers to travel along the path of least resistance. Birds's legs and bodies offer more resistance than the wire they're standing on, and so the current just carries on down the wire. It would be different if the bird were to place one foot on the wire and one on the ground: the current would travel through the bird to reach the Earth - a quicker route than continuing through the wire.
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