There are two main factors that cause me to get thorns in the feet when hiking barefoot:
Usually, the forest ground consists of many layers - grass, beneath it rotten leaves, beneath it grass roots, but still aerated. Thorny plants can be hidden beneath the grass and fully invisible.
One can't always look down where he is stepping, therefore encountering from time to time raspberries or blackberries.
When I hit a thorn, it usually hurts quite a lot in the first moments. However, my feet are dirty from walking and seeing the thorn is difficult. Furthermore, even if I locate it, it is usually embedded deep enough to make removal difficult.
How can I identify which thorns are dangerous in the long therm and should be removed promptly (instead of leaving it inside the foot)? How to determine if the painful thing that hit my foot is a plant's thorn, that will dissolve in the body in a few days, or is a tiny piece of stone/glass, that, if not removed, will embed there and hurt indefinitely long?