Awful, aren’t they! I believe they’re caused by a buildup of grease on the skin and under it at that point – this can also happen around a hair follicle, for example. All skin needs some grease on it to keep it supple, otherwise it would crack from being too dry, but in teenagers there can often be too much grease. (Not only teenagers either – sorry to break it to you, but I still have a little bit of acne and I’m nearly 37! Not as much, though …) The excess grease is what comes out when you squeeze the zit.
Hi @brobdingnagian and @chloetaylorx
They are to do with naturally produced grease (called sebum) clogging up the hair follicles in the skin. The sebum blocks the pore in the hair follicle, trapping dead skin in there, and a spot forms.
Annoying, and like Damien, I still get them from time to time! Grr! They can really hurt too!