system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How Adblock determines what images are ads, and why websites haven't circumvented it yet?
gpt
A majority of advertisements on websites come from the same source websites: Admob, google ads, etc. Adblock acts as a filter and removes these ads before you see the page. Companies can not circumvent this because all the work is done on your end.