The phrase "be thinking" is often used to describe the mental act of considering various thoughts and ideas. However, there are many synonyms that can be used interchangeably with this phrase to convey the same meaning. Some common options include "pondering," "contemplating," "musing," "reflecting," "meditating," "ruminating," "speculating," and "brooding." These words all suggest a level of mental engagement and reflection, indicating that the person in question is actively considering and analyzing their thoughts and ideas. Depending on the context and the speaker's tone, any of these synonyms could be used to effectively convey the same concept as "be thinking".