Don't be a sucker...
You know those signs that you put in the back of cars, 'Baby on Board', that sort of thing... well, they have those suckers that you use to stick them to the window. Once I was in the car and found two window blinds on the rear parcel shelf, they attach to the window with these suckers and block out the sun. Well, I took off the sucker, and stuck it on my forehead. Just for fun. Left it there for about a minute, and peeled it off. I then got the other blind, took the sucker off that one, and stuck them both to me head, still on my forehead, above my eyes. Then I took them off after about a minute and put them on my forehead again, more to the side of my head, and took them off after about a minute. I couldn't see anything wrong with this, I just found it weirdly amusing to stick suckers to my head.
Within half an hour, I had 5, perfectly circular bruises starting to form. This started to worry me but I thought they'd stop. No, they got worse, and in a few hours they were quite clear. Five, very round, purple bruises, in a line across my forehead. It seems the suction had pulled the blood towards the skin, and as a bruise is just bleeding under the skin, that was the result, bruises. The next day at college was interesting. I got funny looks, laughs, and a few people asked what the hell happened. I just sighed, shrugged, and explained that I put suckers on my head.
They were there for at least a week. Some were there for two.
I've put suckers on my arm before to see how long they stay there for, and it's fine if I get a bruise there, nobody's going to see that, and if they do, who cares. A line of bruises on your head is another matter, though. There was a reason I knew I shouldn't put suckers on my head, I just didn't figure out what that reason was until now.