When fingers and toes wrinkle after being submerged in water, it’s not osmosis but rather a reaction triggered by the sympathetic nervous system. Here’s what happens and why:

### The Process:
1. **Nervous System Response:** Immersion activates the sympathetic nervous system, narrowing blood vessels (vasoconstriction) in fingers and toes.
2. **Skin Contraction:** Reduced blood volume in soft tissues causes the skin to pull inward, forming wrinkles.

### Why Only Fingers and Toes?
Unlike other skin, fingers and toes wrinkle because they are key for gripping. This wrinkling creates tread-like patterns, improving grip on wet objects.

### Theories Behind Wrinkling:
– **Improved Grip:** Wrinkled skin helps funnel water away, allowing for better handling of wet surfaces—like tire treads on a road.
– **Evolutionary Advantage:** Some believe this ability evolved to help humans navigate slippery environments.

While the exact reason remains debated, the grip theory is currently the most supported explanation for this fascinating bodily response.