Pneumatic nail guns use compressed air to fire nails. They are fast and efficient, but potentially dangerous if they're not used properly.

The four drawings below show a worker using a pneumatic nail gun. In three of the drawings he is firing a nail into a butt joint.

In the other one, he is freeing up a jammed nail with a screwdriver. But only one of these drawings shows a safe work practice.

The other examples are asking for trouble! See if you can select whether the activity is safe or not.