Today, March 20th, is the International Day of Happiness. You're forgiven if you did not know this, as it's not the kind of holiday that gets folks out of school or work, nor does it involve exchanging gifts. Really, it's just about being happy and encouraging others to do the same, along with coming up with ways to eradicate poverty and establish peace on Earth.
So if you say Happy International Day of Happiness into Hangouts, a little guy will dance out from the side of the screen and strike a happy pose.
After that, he continues to dance until he disappears in the opposite direction. It's a brief animation, and it doesn't exactly radiate happiness the way a thousand smiley emoticons raining from the top of the screen probably would. But it acknowledges the day's existence, which is more than most people will do.