system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5 How does reverse image lookup work?
gpt
There's not a single way but the general idea is to first downscale the image to something small (like 32x32) and then use an algorithm that detects disctintive features of that image and encodes them into an even smaller value, it's like some sort of extreme compression. Comparing this encoded values is much quicker since they're much smaller than the original image.

Also the search engine is continually looking for images in the web in order to keep a database of encoded images and the corresponding addresses. So when you give it an image for a reverse lookup the engine encodes the image in the same way, then looks for similar values in the database and finally obtains the images from the stored addresses.