system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5:How can dogs smell cancer?
gpt
The current theory is that cancer cells produce characteristic organic compounds which get into the bloodstream and are then also present in the patient's breath - only in tiny amounts, but enough to  be detected by a dog's amazingly keen sense of smell.

Which compounds those are exactly is still being researched, in the hope of using the knowledge to produce test kits.