If you are fixing a flat on your bicycle tire, and you can not see what caused the flat tire, make sure you run your hand along the inside of the tire to check for any remaining sharp pieces. Often if a flat is caused by a thorn or something like that, it will break off on the outside of the bike tire, but remain on the inside. If it's still there when you put your new, or newly repaired, tube into a tire like this, you'll be fixing another flat tire on your bike very soon.