Bitcoin

So I decided to buy some bitcoins because it was cool. The exchange rate at the time was around $0.80 per bitcoin. I put in a bit over $40.

Over the last few weeks, the price has nearly quadrupled to $3.80 per bitcoin. My $40 has turned into $190. About 2 weeks ago Paypal decided to ban bitcoins from their system -- this 'caused the price to double. Why?

They keep a daily graph of the exchange rate on this site:

http://mtgox.com/