Could a third factor explain both changes?
Two things moving together do not automatically show that one causes the other. An outdoor kiosk might sell more cold drinks and more hats on the same days because hot weather increases demand for both. The relationship is real, but the explanation may involve a shared cause rather than a direct link.
Go deeper
Look for a common cause
Suppose a kiosk records higher hat sales whenever cold-drink sales increase. The owner might conclude that selling more drinks makes people buy hats.
A different explanation is that hot, sunny weather affects both purchases. The observed association alone cannot distinguish these stories.
A third factor that influences the supposed cause and outcome can confound the interpretation. Checking the relationship within similar weather conditions would be more informative than simply comparing all days together.
Even that comparison may leave other explanations, such as visitor numbers or special events. The point is to ask what else could generate the pattern before treating a useful correlation as evidence of a direct causal mechanism.