In the Science Lab experiment Viscosity of Liquids we examine the properties of different liquids beginning with a comparison of corn syrup and water.
Ketchup becomes thicker, or more viscous, when it sits still.
If you stir it up or shake it, it becomes thinner, or less viscous. We see this by dropping weights through ketchup and timing how long they take to sink down to the bottom. We do this first with ketchup that has been sitting still for a while, and then again after the ketchup has been stirred. Ketchup is a thixotropic liquid. It becomes less viscous when agitated.