We sweat during summer and heavy exercise.

Explanation

(i) Human beings are warm-blooded. We need to maintain a constant body temperature irrespective of the surrounding conditions.
(ii) In summer and during heavy exercise, lot of heat is generated which raises our body temperature .
(iii) Sweating leads to loss of excess body heat and brings down the body temperature to normal.
(iv) Therefore, we sweat during summer and heavy exercise.