There is some skill in blackjack, because the odds change as the cards are dealt. The critical information is the numbers of tens (including face cards) which have been dealt. This is because they are the common cause of 'busting' the dealer hand and because they are needed for a blackjack. Aces are also important because they are needed for a blackjack. Keeping track of the cards is called 'card counting'. It is legal if done without a device like a computer. But, many casinos tell players to leave if they think are counting the cards.