Split the example. Say you are conducting an experiment and your friend walks in in the middle of it. Explain the setup (bag of 10 coins, 9 are fair and 1 has two heads) and that you've taken one coin at random from the bag. Ask what the odds are that heads will come up if you toss it. The answer is what your friend gave:
Now ask what the odds are if you tell him that you've already tossed the coin once? If he's smart, he'll ask you what the result of the toss was. If you answer 'tails', would that affect the odds of the next toss of that coin being heads? Yes, with the new information you know that the coin is one of the 9 fair coins so the probability is now
The odds change if you know what coin