Quality Control
The scientists must make sure each sample is exactly the same and gives the same sun protection. We call this quality control.
They can do this by shining light on each sample, measuring how much light is blocked, and drawing a graph. Thankfully most of this work can be done using a machine!
A good graph should look like this, showing that the harmful ultraviolet is blocked, but ordinary sunlight can still pass through.
On the next page there are four samples for you to test