Well I think it's time you bought it 

You run a fairground. At the entrance people buy tokens - 1 token counts for 2 rides. Inside the fairground, there is only 1 currency: token currency. (Call it Bitcoin if you like).

Each day, you have an average of 100 people inside the gates at any one time, each person buys on average 5 tokens. So you maintain a "money supply" of 500 tokens which are constantly being recycled, even though the ride capacity is not fully utilised.

Then one Saturday, due to excellent weather, you get a 50% surge in visitors. Now you have 3 choices:

1. inflate the prices so that 1 ride costs 1 token instead of half a token
2. turn people away
3. get more tokens (inflate the money supply) and keep prices the stable

Sure, you can have stable prices with a fixed monetary base, but only if demand for liquidity remains stable (which is cannot in a dynamic economy).