What we call “brain teasers” are riddles or puzzles which are designed to challenge your logical and outside-the-box thinking. This may be for example: “Which 7 letter word becomes longer when the third letter is removed?“.
“Market sizing guesstimates” are sometimes referred to as brain teasers, as well. They are more common in case interviews and consist of one question asking to determine a specific number of things, for example “How many golf balls can you fit into a passenger Boeing 747?”. These questions test your ability to define a problem, break it down based on assumptions, and use math to solve it.