A tin of tobacco is too great an opportunity to miss. Tobacco is like gold in the trenches, not only to smoke but also profitable to sell. You decide to risk it.
Making sure nobody sees you, you stand on your toes and stretch one arm towards the tin. It is just barely beyond reach, and you decide to stand on your own helmet to give you some added height.
Your fingers slowly grasp the lid when, out of nowhere, the crack of a rifle rings out and a bullet buries itself deep in your skull, killing you instantly.
No one in your trench saw you reaching for the tin. But the sniper over in the German trench certainly did.