Something that's puzzling is confusing, or hard to understand. It might seem puzzling to you that your best friend claims to hate doughnuts.
People sometimes act in puzzling ways, making decisions that you can't comprehend. You might also come across particularly puzzling questions on a test or quiz — these are the hard ones, taking a long time to figure out and requiring a lot of thought. Puzzling comes from the verb puzzle, whose origin is a bit of a puzzle itself. Some experts guess that it comes from pose, formed the way nuzzle is from nose.