Beer's Law
Beer's Law is the rule that light gets weaker as it passes through more of a colored liquid, like sunscreen blocking UV rays.
Think of it this way
A glass of dark grape juice blocks more light than a glass of lightly colored lemonade. Hold each glass up to a window and look through it. The darker the liquid, the less light passes through. Adding more coloring to a glass makes it block even more light, the same way a thicker sunscreen layer blocks more UV rays.
Explaining beer's law by grade level
Think about putting sunscreen on your arm. A thin layer lets some sunlight through, but a thick layer blocks more. That is how colored liquids work too. The darker or thicker the liquid, the less light passes through. You can see this by holding a colored drink up to a window.