Problem from Dayton's Catalog

Yesterday I told you the story of finding a picture in a catalog which had a grandpa playing chess agains his grandson. The grandpa had only one pawn left while the youngster still had all his pieces. However, when I examined it, I realized the grandpa, who was getting ready to move was actually going to win the game! (The actual position in the magazine is unknown, but it was siimilar to this one.) How does grandpa win?