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.
Figure: Okra (Abelmoschus esculentus)
The video below shows what happens when the fluid inside Okra is stretched. Note the formation of stringy fibrils.
Video: Stretching Okra mucilage.