Why do we preferably have round manhole covers and not square ones?

On a special event at the prison, 5 prisoners John, Jacob, Krist, Tang, and Dang given a chance to go to a sauna and can bring one thing with them.

Following are the things that the prisoners bring with them.
John: A soda
Jacob: Thermos
Krist: Book named "How to kill"
Tang: A Walkman
Dang: Harmonica

Since it is a Sauna, no one can see each other. After a while When the steam is switched, Tang was found dead and there was blood all around.
J Bond was called and he instantly come to know who can be the murderer. How?

By removing one matchstick and adding one matchstick, can you convert the word "ZOO" to an animal name?

Two brothers were watching a horror film on video late one night. One brother dozed off and dreamed that he was being chased by the crazy man from the movie, who was trying to kill him. In the dream, he hid in a cupboard. There was no sound except his heart pounding, and he had no idea where his crazed captor was. He was terrified! At that moment, the video finished, and his brother put his hand on the shoulder of his sleeping sibling to wake him. The shock at that tense moment was enough that the sleeping brother suffered a massive heart attack and died instantly. True or false?

Two in a corner, one in a room, zero in a house, but one in a shelter. What is it?

What brings you down but never up?

Why is 6 afraid of 7?

What does this simple rebus mean?

What breaks yet never falls, and what falls yet never breaks?

You need to move three matchsticks to form three squares. Can you do it?