Mirage appears in the desert. Give reason.

When a pencil is partly immersed in water and held in a slanting position, it appears to be bent at the boundary separating water and air. Give reason

Symptoms of disease appear only after a certain period of mosquito bite in case of diseases spread by mosquitoes. Give reason.

Velvety green beds appear on the walls during rainy season. Give reason.

Give the location of testes in human male. Give reason.