Trappd skin fragment. Due to a minor injury, pimple, etc., a piece of skin gets trapped under the normal skin. It doesn't know it is not on the surface, and so it makes oil and hair and it sheds dead skin normally. All of this accumulates under the surface and forms a small cyst. As more debris is added, the cyst gets bigger and bigger. They are recognized by their overlying enlarged pore and cured by excision.