BrainTeasers are a frequent type of questions asked in trading interviews. A brain teaser is a type of puzzle/riddle that in order to be solved will need a significant amount of thinking, brain power and a structural approach. The reason that they are so favourable in trading interviews is because you can demonstrate your way of thinking. Even though finding the correct answer it is important to demonstrate and think out loud the way you have reached to the answer.
The best way to prepare for them is to practise of course. The method to practise is to read questions and try to find the answer. Try to get familiarised with some type of questions, and also developed your own structural methods of thinking.