Sarcoma!

My attending presented me with a two-foot long segment of colon. While I looked at it, the attending pimped me, "So, what's wrong with this colon?" I then realized the colon was twitching and moving around in my hands and knew the answer straightaway, as only one tumor had enough muscle to keep twitching after it had been removed from the patient. I exclaimed, "sarcoma!", and was right.

(Note: sarcomas do NOT twitch in real life.)