Many gels and colloids are thixotropic materials, exhibiting a stable form at rest but becoming fluid when agitated. Some clays are also thixotropic, with their behavior of great importance to Structural engineering in Earthquake zones. Clayey ground can practically liquefy under the shaking of a tremor, greatly increasing the effect on buildings.
The classic example of a thixotropic fluid is Ketchup, where waiting for it to pour can be more effective than pounding on the bottom of the bottle. Thixotropic compounds are important paint additives, allowing a thick, smooth application that doesn't run.
Fluids which exhibit the opposite property, in which shaking for a time causes solidification, are called Rheopectic and are much less common.