Digress (v) - to wander off a subject (talking or writing). "When I make jokes in a speech about another subject, I digress." "Michael started his story about a rabbit, but soon digressed into a tale...
I can give you several sentences. Try not to digress from the topic. But, I digress; we were talking about sentences. He tends to digress if you don't remind him what you are talking about.
Someone asked me to use a certain word in a sentence, but my mind started thinking about how little is known about the origins of certain words, how they evolved, and how they are commonly used, but...