Q: Why do hamburgers change shape from round to oval when barbecued?
(there were several contributions - see the full New Scientist article at 12 Aug 2006)
This was my contribution to the discussion:
A: Anisotropy does not only occur in burgers. It also occurs in many rolled or extruded products. For instance, newsprint tears much more easily along the grain, and swells when wet, but only across the grain. The plastic used to make carrier bags is extruded and stretched and so also tends to have a preferred tearing direction. This should be aligned vertically so that a loaded bag doesn't