The Glass of Water Theory is a recommendation that when someone asks you for something you want, just say yes. Do not feel uncomfortable responding "yes" to an offer. This is referred to as the Glass of Water Theory because of the common debacle many of us find ourselves in. When entering a home if someone asks you if you want a glass of water, say yes, otherwise you don't get the water - and they don't think you are being extra polite either.
"Jake's mom offered to pay for dinner. I feel bad letting her, but the Glass-of-Water-Theory states i should just allow her. Now i get a free dinner!" :)