Anonymousjust tell me dont have the time to watch it
Well, since you asked so nicely. For those who haven’t seen the episode and don’t want to know, read no further ;)
Basically this is how it went (this is from memory so things might not be exactly right) Leonard was trying out an experiment with some rocket fuel in the apartment. He messed up the formula and it was going to explode. Leonard got in the elevator to bring it outside, but Sheldon took the “bomb” from and put it in the elevator by itself. Just as the doors closed it exploded, destroying the elevator.
It’s a really great episode. I highly recommend it :)