what looks both ways

From Sounder by William H. Armstrong:

He looked out of the window too. “If you’re inside you look out, and if you’re outside you look in, but what looks both ways? That’s a riddle; what’s the answer?” …No one answered. “What’s the answer?” the boy repeated, and then he answered his own riddle. “The window is the answer; it looks both ways.”

This passage brought to mind the description of icons as “windows into heaven.”