How are odds determined by Bookmakers?

If a bookmaker offered odds of, say, 6/5, it would always be to the bettor’s advantage. You would then bet, say, £50 on ‘heads’ and £50 on tails and you would make a profit regardless of the result. Bookmakers are, of course, far too smart to let this happen. However, it is possible that comparing odds at bookmakers can create such a win/win situation.