What is that red spot?

When watching the kids one night while I was working, Bob was trying to entertain Stacy in her high chair.  He had fed her and she wanted to get down, but he wanted to clean up first.  She had a toy that was suction-cupped to her tray.  It was the only toy she couldn't get loose.  Bob did and entertained her for a while.  He was putting it on and off his forehead.  The next day at lunch, his fellow workers were looking at his forehead and wanted to know what was wrong with it.  He had a perfectly round red mark in the middle of his forhead.  At first he didn't realise what it was.  All of a sudden it hit him and he started to laugh.  He explained to his fellow workers that it was done by a suction cup on the baby's toy.  He tried to hide it but couldn't.  By the time he came home it was dark red and very prominent.  Good thing he wasn't going anywhere cause it really looked funny.  He never did that again!....