Why does nail top coat thicken in the bottle before it is all used up?

Nail top coat thickens in the bottle due to the evaporation of solvents over time, especially when exposed to air. As the liquid evaporates, the remaining ingredients become more concentrated, resulting in a thicker consistency. Additionally, exposure to light and temperature fluctuations can further accelerate this thickening process. Proper storage and sealing can help prolong the shelf life and maintain the desired texture.