Why does a bottle of tomato sauce have to be shaken vigorously to make the sauce flow out?
Some liquids are like a jelly until they are disturbed or forced to move. Then they flow freely. The faster they are made to move, the faster they flow. There’s a name for such liquids. They are called thixotropic. Tomato sauce is thixotropic. When you shake the bottle vigorously or hit the sides, the sauce becomes less firm and starts flowing.
Some paints are thixotropic. They do not drip when they are on the brush but when they are moved along the surface of the wall with the brush, they become slippery liquids.
Thixotropic liquids are found in nature too. Quicksand is one example. The more one struggles in quicksand the less firm it becomes, and the victim is eventually drowned.