OK, I just got back from Philly for the weekend. And as we were landing, I looked out the window, and saw that the runway had a million tire skid marks on it. I guess everytime a plane lands, when the tires 1st touch-down on the runway, the tires must skid. I wonder how often they must have to replace these tires. And what if one just explodes? What happens then? I wonder how often that happens?