Fireflies produce green-yellow lights in their bodies. Light is the means of communication for fireflies. As well as giving mating signals, this light is also used for defense against enemies. By means of light, their enemies understand that fireflies are not tasty and thus decide not to feed on them.
As the special diving tanks in a submarine are filled with water, the vessel becomes heavier than water and sinks deep down in the ocean. If water in the tank is emptied and replaced by pressurized air, the submarine rises to the surface of the water again.
A fish expends a large amount of energy while swimming. This is not because it swims fast for long times. Fish need considerable amounts of energy in order to reach high speeds from being stationary. It is vital for a fish to gain speed instantaneously because it needs to do so in order to escape from predators.