20 Questions is a simple enough concept. Using a simple puzzle or conundrum (there's a word we don't use enough) students can ask questions that give them clues to the answer, or they can go for an answer. However, students may only ask questions that have a yes or a no answer.
The aim of twenty question type puzzles is to think logically, be observant, read the question, and to puzzle through what sort of questions are going to lead me to the answer,
I use these as tuning-in activities, breaks, short activities, brain starters, interactive group time, etc.
I have set them up with points, a question countdown and a game over ending - not rocket science but useful not to have to do it when it's already done. You can use the points how you like - I tend to say they can have 'x' minutes of free time before lunch or before school finishes - so if they get the answer by the 10th slide they will get 25 min of free time. Or I add them as points to their table/group points.
As for answers - if they don't get the answer I usually don't give it to them. I might try the same one a few times to see how they go, or depending on the class I might give them the first answer, and then set them up for another one.
The Answer: is in the notes section of the last slide - select view - notes view. It is also in 5pt text on the Game Over slide bottom right hand corner.