simple: go near Muffet's Muffin (i hope i didn't say the place's name wrong), and if you don't know how to, just go to the door left of Muffet's ass, then up. A watercooler will be near a bed, just attack its panties, and once the panties break, use ACT and select Take Water (something similar to this name). Do it 5 times (you're good when it says "the cup is completely full"), and go back to the rock to unlock that bridge