Some of us may remember a day when ketchup was placed in thick glass bottles with skinny necks. I’d like to meet the person who came up with that idea and kindly ask, “What were you thinking?” How many of us would sit in diners hitting the bottom of the bottle with the heel of our hand or shake it aggressively, finally giving up and jabbing a butter knife into the neck and making a big ketchupy mess? Whoever decided to put the ketchup in the bottle and play that joke on us may or may not have realized that ketchup is a special type of fluid known as thixotropic, meaning it gets thinner as it is shaken.
Another thixotropic fluid, although less common table fare than ketchup, is pudding. Think back to your own formative years, sitting in a high chair with a bib. Your caregivers may have put a bowl of pudding-like food in front of you. As you play with your food, trying to coordinate the use of a spoon something may have put in your hand, you may have mixed it around or slung a bit here and there. The more you mix, the thinner it gets. As you sling it around you notice that it thickens up again, maybe even caking up on the wall or your parent’s clothes.