You may have seen certain questions more frequently than others and wondered why that happens. Those are probably backstop questions.
The poll unit loads engagement questions first, followed by one or two value questions, and then a profile question. If a question is pinned to the page, it is loaded first. If there is no pinned question, an engagement question from a feed is served first.
If a respondent stays on the page answering questions repeatedly, they may come to the end of the available engagement questions. In that case, a backstop engagement question will be asked. (Backstop questions are engagement questions. They're simply lower priority than others.) If there are no backstop questions either, a value question is served.
Backstop questions can be re-asked at varying intervals from 0 to 30 days.
Examples of backstop questions include: How are you feeling today?, Do you subscribe to a print newspaper?, and Would you say you've seen something you deem to be 'fake news' / 'fake media' this week?