FX Quoting: Bid/Ask and Spread

There are sometimes that you can only see one price but often currency exchange price are display in pairs with 'bid price and ask price'.
For example EUR/USD 1.2385/1.2390, 1.2385 is known as the bidding price, while 1.2390 is the asking price. Bidding price is the price that you sell the base currency (EUR in our case here); asking price is the price that you buy the base currency. The different of the bidding and the asking price is called 'spread'.
You might notice that bidding price is always lower than the asking price. Ever wonder why? The different of the bid-ask price (socall 'spread') is how currency brokers make profits without charging commissions to their clients (sell high and buy low in the same time.)