It is a bad comparison on the grounds that in the example of a grain of sand you are exchanging something that is abundant and widely available for a precious good, whereas, in the case of bitcoin, you are exchanging something scarce and hard to obtain for the same precious house. It is true that only irrational people would exchange a valuable thing for something they could get for free. Apparently, you can't believe and understand that something like bitcoin, "a number," can be scarce and valuable, and you are calling other people greedy, stupid or irrational because you can't understand why they keep exchanging valuable things for something you consider worthless. Either you are defending your ego because you can't admit that other people are smarter than you or you are smarter than millions of people who mistakenly believe that digital scarce decentralized money is not nonsense and that the only justification for the existence of traditional money is that governments have managed to coerce people to use costlessly-printed money in their transactions. In either case, you have no choice but to accept the fact that bitcoin is used and seen as money by other people. No matter how smart or stupid those people that have chosen bitcoin, bitcoin will stay and will function as intended because it doesn't even care what people think about it. You either buy it or ignore and resist it while you can.