Correlation

Have you noticed that the stock price of some companies will be closely followed by another, usually a rival in the same sector? The theoretical explanation is that because these two companies are in the same type of business, they share the same challenges, require the same materials and resources, and compete for the same type of customers.

You could think of many possible pairs, but you need to check for a real relationship. One way is to take a look at the correlation of the stock returns of both stocks (see https://www.khanacademy.org/math/probability/statistical-studies/types-of-studies/v/correlation-and-causality). A high correlation implies a relationship of some sort. It is not proof of causality though, especially if you don't use sufficient data.