All the air is removed in a sealed metal can. after the air has been removed, the can looks as if it were crushed. why?

The can looks like it was crushed because the outside pressure (aka atmospheric pressure) is pressing against the can that has no air in it, which is also called a vacuum in chemistry terms. The main thing to focus on is the atmospheric pressure that is pressing against the can and in the process, crushes it.

in 1859, charles darwin set out his theory of evolution by natural selection as an explanation for adaptation and speciation. he defined natural selection as the "principle by which each slight variation [of a trait], if useful, is preserved".

answer: ice crystals in air

the water from the water body or another source of water gets evaporated as water vapor as an effect of heat on water. these water vapor rises in the upper layers of the earth atmosphere under high atmospheric pressure. the atmospheric pressure of these water molecules starts to decrease due to the effect of cold air. the invisible water vapor condenses into visible water droplets and ice crystals.these ice crystals and water droplets being less denser than the surrounding air floats on air as clouds.