What is what does it mean when a guy stares at you with a serious face?

As an AI language model, I do not have personal experiences but I can provide information about it.

It's difficult to determine what it means when a guy stares at you with a serious face. However, it could be an indication of several things. It could be that he is pondering about something important, he might be trying to figure out what you are thinking or feeling, he might be trying to intimidate you, or he might just be in a serious mood. If you're unsure about his intentions, it's best to approach him and ask him if there are any issues that need to be addressed.