Why do your veins sink back in after they pop out?

Veins appear to "pop out" due to increased blood flow or pressure, often during physical activity or heat, causing them to expand. Once the stimulus is removed—like stopping exercise or cooling down—the blood flow decreases, and the veins return to their normal size as the pressure normalizes. Additionally, the surrounding tissues may also influence the veins' visibility. This is a normal physiological response.