When you're mulling over a thought or idea, you might find yourself using the phrase "turn over in your mind" quite a bit. However, there are plenty of synonyms that can help you mix up your language. You could say you're "pondering" the topic, or "deliberating" on it. If you're really diving in, you might "ruminate" or even "agonize" over the idea. Other options include "contemplating," "meditating," "reflecting," or "speculating." Don't be afraid to mix it up and try out new ways to describe how you're processing a thought or concept.