Here's why:

* Concentration is defined as the amount of solute per unit volume of solution.

* When solvent evaporates, the volume of the solution decreases.

* Since the amount of solute remains constant, the decrease in volume leads to an increase in concentration.

Example:

Imagine you have a sugary drink. If you let it sit out in the sun, some of the water (solvent) will evaporate. The amount of sugar (solute) stays the same, but the overall volume of the drink decreases. This means the drink becomes more concentrated, or "sweeter."

In summary: Evaporation of solvent leads to an increase in the concentration of the solution.