What does that really mean?

Recently, Anthony came up to me and said, "We broke the window sill!" I was reasonably certain that they couldn't have broken the window sill, but that left the question of what had "broken" that they thought was called a window sill. I was hopeful that they hadn't actually broken glass since I hadn't heard shattering glass, but I still came to check it out. The "window sill" was actually a curtain rod that they had knocked down.