If one had to activate the fire alarm in a fire how do you smash the glass?

On a fire alarm in the UK where it says”Smash Glass” Can you kick it or do you hit it with your hand? Why is there a place to press with your thumb? Who would smash it with their hand and who would push it with their thumb? Do you know anyone that ever had to activate one? Can it take two people to smash the glass?