|Many polymers and non-Newtonian fluids occur naturally. Examples include
DNA, saliva and tree sap.
Okra (Abelmoschus esculentus) has been used for thousands of years as a thickening agent in foods, thanks to the high molecular weight polymer that it contains.
Click on the image to play a video (400KB) that shows what happens when the fluid inside okra is stretched. Note the formation of stringy fibrils.