system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How do credit card rewards work, and can I really get free flights and such just by spending money?

I only have one credit card right now, which I am using to slowly build my credit by buying gas each month (anywhere from $75-$125 a month). I do not really know anyone who uses a reward card, but I've heard plenty about frequent flyer miles and other rewards and such. How do these cards really work?
gpt
Credit card companies make money in 2 main ways; 

1) If you do not pay of the balance at the end of every payment period (usually a month, but there might be cards with other terms), they can charge interest. Usually this is a rather high rate.

2) Companies that want to accept payments from credit cards must pay the credit card companies for this. For this reason credit card companies give you gifts for using their cards. If you want those rewards, you'll use your credit card more often, which means companies must pay your credit card company more.