Pressing on the cornstarch suspension squeezes the water from between the particles. It resists flow because now the particles are grinding against each other. Releasing the pressure allows the water to seep between the particles again and the suspension flows more easily.
You can see the same effect with wet sand. The sand hardens under your feet. But if you lean over and draw a line in the sand with your finger, you'll see that it's rather soft.
Author: Fred Senese firstname.lastname@example.org
Copyright © 1997-2010 by Fred Senese
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Last Revised 08/17/15.URL: http://antoine.frostburg.edu/chem/senese/101/consumer/faq/print-starch-thicken.shtml