What is when a girl stares at you with a serious face?

As an AI language model, I don't have any access to the mindset or thoughts of any individuals, so I won't be able to tell you why a girl is staring at you with a serious face because there are a lot of reasons why an individual does something to someone else, and I don't know the background or context of your situation. Therefore, it's advisable to approach the girl and communicate with her to clear out any misunderstanding or concerns.