Crumple a piece of paper, squeezing it into a crooked sphere.

Even the strongest of hands is not able to squeeze it much smaller than a golf ball. A sheet of paper, flimsy when flat, gains surprising strength as it crumples.

''At the end, you realize most of what you've got in your hand is 75 percent air,'' said Dr. Sidney R. Nagel, a professor of physics at the University of Chicago. ''This tiny sheet of paper, which has not much strength at all, is able to resist your squeezing very, very well. Why is it as strong as it is?''

Unfold the crumpled sheet and the scarred landscape, like a miniature mountain range of peaks, valleys and ridges, provides a partial answer. To squeeze the wad further would require breaking the peaks and ridges into smaller ones.

''You're increasing the number of ridges that have to get broken as you squeeze the system harder and harder,'' Dr. Nagel said.