This is an activity that can get your mind going or simply get creative. For example I could give a simple sentence and expand it without using the specific sentence I gave you. You have to be telling the story or the scene and it has to be in the present.
Example:
The heat was suffocating.
The paint was peeling off the walls and hanging oddly at sides. The house creaked with effort and seemed ready to fall at any moment. As I ran past a mirror glued to the wall I caught beads of sweat were running down my forehead and disappearing down my shirt. Panic and frustration were visible in my eyes and my hands wouldn't stop shaking. Suddenly the glass began to bend in and shoot straight out towards my face. I dropped to the ground in an effort to cover myself from the flying shreds. The air down near the ground was more breatheable and not filled with smoke. I looked up and tried to see the end of the hallway, but was met with a mirage instead. Any moment now the flames would lick away the only safety I needed to escape this trap.
Something like that. Try to not go over five minutes in writing. Just write whatever you can think of. The next person can continue with the sentece.