Like a tune, but much simpler. I almost always have one. It’s “Dardanelles” for me today (“a narrow strait in northwestern Turkey connecting the Aegean Sea to the Sea of Marmara”). I read the word in a book yesterday. Today, I’m walking around thinking “Dardanelles, Dardanelles” between other thoughts. It will keep going until it’s replaced by another one, perhaps “uvula” or maybe “parlous.” But until then, it’s Dardanelles Dardanelles Dardanelles!