That may be true, but I don't see that as the main purpose of a warm-up.
In my classroom, a warm up or bellwork is a key part of instruction. I use them as daily quizzes, worth 2 points each day. At the end of the week they get added up for a ten point quiz grade. They are graded 2 points if correct, 1 point if tried but partially correct, and 0 points if not tried. I used to actually grade them for correctness, but I really want the students to try, even if they are unsure, and they will get 1 point for trying. I want to use bellwork more as formative assessment than summative.
You can see the bellwork sheets that I use on a weekly basis here. This really saves me time at the copier! Instead of copying daily bellwork, I copy once a month and then put the bellwork on the board or smartboard when they come in. They just record their answers in the daily box.
I choose the questions that I use for bellwork very carefully.
At the start of the unit, I make bellwork very open-ended. Some examples are interpreting a diagram, or making observations about a diagram. I have also had bellwork early in the unit that focuses on a small reading passage, a connection between last unit and the new content, or even listening to a song or watching a video and stating what they think it is about.
I also like to use bellwork to build prior knowledge, if possible, or to practice basic skills, such as determining what is wrong with a graph, or to learn new vocabulary.
As the unit progresses, I ask content based questions, usually from what we just did yesterday. This is where bellwork holds them accountable for yesterday's work, gives them a heads-up if they were absent, and gives me a quick look at understanding from yesterday. At this stage in the unit, I usually use concept-based questions, but write them in my own words, or in more student friendly language, although I may use some diagrams from state exam questions.
As we get closer to finishing up a unit, my bellwork will mostly consist of state test questions on the topic that we are studying. This is where they really get used to the language of the exam, and apply what they have used.
Lastly, depending on the group of students that I have, I sometimes do bellwork in a different way. I teach in an urban district where reading levels are very low. There have also been years where I have had a lot of ELL's. Many times they are struggling with not only unit concepts and vocabulary, but tier 2 vocabulary, which we don't traditionally teach in science class. I have used bellwork as a chance to teach some of these tier 2 vocabulary words, using these products in my store. These give students pictures of a word, and ask them to infer the meaning. We worked on 4 or 5 words a week, and then had a quiz at the end of the week. This really helped, and made students more confident with inferring meanings of unknown words as well.