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:
In a vacuum, air pressure on a sphere. This would make the chicken lay eggs.