Laughing at Leonard's chicken joke.
Leonard told Raj, Penny and Howard his opening joke for his talk on Sheldon's and his paper on Paradoxical Moment-of-Inertia Changes Due to Putative Super-Solids in The Cooper-Hofstadter Polarization.
The Joke:
"A farmer has some chickens who don't lay any eggs. The farmer calls a physicist to help. The physicist does some calculation and says "I have a solution but it only works for spherical chickens in a vacuum!".
When Sheldon crashed the conference, he said that it was a good joke
Explanation:
When learning physics, to make things easier and clear, objects are usually spheres or squares in a vacuum (this make the problems easier to calculate). As a result, the physicist in the joke says his solution "only works for spherical chickens in a vacuum", making it an inapplicable solution, but funny because it's truly the way you learn physics.