Best example I ever saw was doing my family practice rotation. Patient was complaining they'd gained 10 lbs. Physician asked "You add a can of soda to your lunch or dinner last year?". They said (paraphrasing) "Yes lol how did you know". They did the math. Keep everything constant. Add ONE can of soda a day. One year will add 10lbs. Was mind boggling for the patient and me at the time.