Your a human who just recently fell in the underground as you were about to walk on the bridge in snowdin but then a skeleton appears behind you and tells you to turn around.

Sans: * Heya don’t you know have to greet a new pal? Turn around and shake my hand.

you shake his hand and you hear a fart noise. Its sans’s old joke.. The whoopie cushion. As you would let go and see a pink cushion in the hand as the skeleton would laugh