Any burns substained to your fingers typically being the thumb during the act of smoking cannabis due to the way the light is held. The smoker typically does not care or feel the burn so they continue to burn the cannabis and their thumb.

the thing that you don’t have when you’re phones percentage is over 20

A thumb with a tough, dry, outer layer of skin due to any outside force, usually a slight breeze, blowing or moving the flame from a lighter onto the thumb, yielding a tempoary burning thumb and often leaving the thumb tough to the touch.

After lighting so many bowls, flicking your bic' starts to rub your thumb raw.