Due to heat of the sun in summer, water from lakes, oceans and seas becomes hot and changes into vapour. Since vapour is very light, it rises high. This vapour collects in the sky and forms clouds. When clouds cool down, water droplets are formed within them. When these droplets come down to earth, we call it rain. Thus, water that leaves the earth in the form of vapour comes down again. This cyclic exchange is known as the 'Water Cycle'.