That’s a perfectly valid question. The answer is very interesting:

https://www.awm.gov.au/articles/encyclopedia/gallipoli/drip_rifle

Drip (or "pop off") rifles were self-firing rifles used at Gallipoli to deceive the Turks during the evacuation of December 1915.

Fire was maintained from the trenches after the withdrawal of the last men, by rifles arranged to fire automatically. This was done by a weight being released which pulled the trigger. Two kerosene tins were placed one above the other, the top one full of water and the bottom one with the trigger string attached to it, empty. At the last minute, small holes would be punched in the upper tin; water would trickle into the lower one, and the rifle would fire as soon as the lower tin had become sufficiently heavy.

Another device ran a string, holding back the trigger, through a candle, which slowly burnt down, severed the string, and released the trigger.

Iirc, they were left behind when troops moved out. The drip doohickey would make it fire after a while, giving the enemy the impression the trenches were still manned.