system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How does counting cards work?
gpt
In blackjack, you are much more likely to beat the dealer using a deck that has more "big" cards (aces, face cards and 10's) and less "small" cards (2's, 3's, and 4's). One of the reasons for this is that the dealer *must* take another card when he has a hand of 16 or less, so the more high cards there are left in the deck, the better his chances of going over 21 and losing, while a lot of low cards means he has a good chance of getting a very strong hand, like a 19 or a 20. Another reason is that a lot of aces and 10's mean more blackjacks (two-card 21's) for players, which pay out more money than just winning the hand. Because of this, if you know there are a lot of high cards in the deck, you should bet more, and if you know there are a lot of low cards in the deck, you should bet less.

The easiest way to count cards is to just keep one number in your head. When a new deck is used, start at the number 0, and watch every card as it is played. Every time you see an ace, a face card or a 10 played, subtract one from your count. Every time you see a 2, 3, 4, 5 or 6 played, add one. That way you keep a pretty good idea of what kind of cards are left in the deck. If your count is at -6, for example, you know that there are more small cards left in the deck than big cards, so you should make a small bet. If your count is +8, you know there are many more big cards left than small cards, so you should make a very large bet.