When a clock controlled by a pendulum is taken from plains to mountain, it becomes slow but a wrist watch controlled by a spring remains unaffected. Why ?

Explanation

This is due to decrease in value of g at the mountains. The time period of pendulum clock increases with the decrease in value of g. Thus, the clock slows down. On the other hand, wrist watch is controlled by spring, whose time period is unaffected by the variation in g.