Just to test the brilliance of Sherlock's mind, Dr. Watson makes five people stand in front of him. Out of the five, only one is the truth teller and the other four are togglers which means they may tell the truth or lie on being asked. But on being asked again, they will switch which means if they told a lie the first time, they will tell the truth on second question and vice versa.
Sherlock is challenged to ask only two questions to determine who the truth teller is. He can ask both the questions to the same person or ask two different people. How will he determine who is the truth teller?
Solution:
Sherlock will ask any one of them "Are you the truth teller?" There can be two responses to this question - Yes or No.
If the person says yes, then he is the truth teller or a lying toggle.
He will ask the second question to the same person "Who is the truth teller?"
If he is the truth teller, he will admit it. If he is a lying toggler, he will speak truth on the second question if he lied on the first, telling you who the truth teller is.
If the person says no, Sherlock will be confirmed of the fact that he is not the truth teller and thus he is a toggle who has told truth and will say a lie on the second question.
The next question he will ask to the same person "Who is not the truth teller?"
Since the person told truth in the first question, he will speak a lie this time and in order to tell a lie, he will tell Sherlock exactly who the truth teller is.