You can see or feel most properties of a sheet of paper. Its color for example, its size, its thickness, and its weight. But the grain direction (the direction the paper fibers are aligned to) is invisible to the eye and not easy to feel by hand.

A sheet of paper folds easily along the grain direction. If you fold it across it instead, it creates an uneven crease. A thick enough sheet will even crack and won’t lay flat anymore.

So, sheets of paper are best folded with the grain.

I recognize that it’s a pretty useless piece of information unless you’re into bookbinding, origami, paper planes, trivia, or metaphors.