What are Sam and Carol doing?

M:Carol, look at this picture. When I first saw it, I thought it was a picture of a candle.
W:Is it a picture of a candle? Oh, no! It’s a picture of two people looking at each other.
M:It seems to change while you look at it.
W:Yes, here’s another one. Which line is longer,Line AB or Line CD?
M:Let’s see.Line CD is longer.
W:You are wrong, Sam! They are the same length.Line AB looks shorter because it’s interrupted by Line CD.