A reference is critical because it determines how much motion has occurred. You can't determine how much an objected has moved if you don't know where it came from.
Reflection. It occurs when a wave bounces from the surface of an obstacle
Easy ! EVERY element and every compound melts, then boils and becomes a gas, if you heat it to a high enough temperature. That includes iron, gold, water, salt, glass, almost any substance.
-- sometimes presented as a "perpetual motion machine" /// -- impossible to construct /// -- a scam /// -- a fraud /// -- often lubricated with snake oil
Customers are willing to pay what they think something is worth and don't really care about your costs. If your costs push prices above their perceived value, they simply won't buy. If the perceived value is much higher than your costs, they'll happily pay a price that gives you a huge margin. One of the best examples of this is in retail clothing.
