Farrier trying a shoe.
After trimming the hoof the farrier heats the new shoe & burns it on the hoof to check the fit is good.
It doesn't hurt the horse as there are no nerve endings in the hoof (As our toe & finger nails).
If the fit is good the farrier will cool the shoe before nailing it on.
http://www.youtube.com/watch?v=s8WpHMPNiRk