Chronologically episode 5.  While walking home with Haruhi she tells Kyon about why she has done so many strange things.  It all happened when she was in sixth grade and went to a baseball game.  She thought she was big stuff in her class but when she saw all of the people in the stands, she realized that she wasn't all that important after all.  She then realized how small that crowd was compared to all of Japan and it made her feel even less significant.  That's why she feels the way she does.  Not able to respond, Kyon heads home.  Itsuki is there waiting for him and takes him to a place where he can show him his esper powers.