A little background... Earlier in the episode, Sheldon planned to have multiple presents ready for Penny so he could give her a present equal in value to the one she gave ...
Season 2, episode 13 Sheldon displays his friendship algorithm as a flow chart, and tests it. (this belongs to CBS, not me, I'm just enlighting you with Sheldon's awesomeness)
A fan made vid no copy right infringement intended. Lyrics: Our whole universe was in a hot dense state, Then nearly fourteen billion years ago expansion started. Wait......