The response in this thread is ridiculous.
I see only one way for your case. If you tried to explain bitcoin to your brother and he didn't get it, must be he is just not interested at all or you used some hard and professional way for explaining bitcoin.
My advice - ask him for this 50$ and promise him to bring him back 100$ in some time. That's it.
Wish you luck.
OP was asking for an explanation to make his brother lend him $50 and your answer was to ask him for $50.
You do not have to explain bitcoin as an alien to your brother. You only tell him that you are starting a website and for that you need some money to buy a domain and to buy a hosting server online. So he will pay you that money and then you can convert them to bitcoin and when he want to inquire the way you are going then tell him that it is online currency and for buying domain and hosting etc you have to convert these money to bitcoin currency.
You don't think he's going to ask what it's about? If he asked, OP would say it's about "bitcoin" and his brother would say "What's bitcoin?" We're back to square one.
Bitcoin has many advantages, but you have to be familiar about it before explaining about it to your brother. Why don't you create a blog,write a content about bitcoin and start marketing on free services like facebook and twitter. Once after increasing your alexa rank go for advertising options, in this you will make more money through advertising.
That's exactly what he need the money for.
Well, so much to gain bitcoin. if you want to explain it, you just need to say that to start a business with bitcoin, you do not need capital, most importantly, you're at it well. besides, if you want fast, you just need a little money, and you can turn that into a lot of money if you are already proficient in bitcoin trading.
Don't you think it would be ironic saying that "One does not need capital to start business with bitcoin." while asking for a loan of $50 so you can start your bitcoin business?