system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5, Why are spiders unable to crawl on another spider's web?

I noticed this while cleaning my windows. A spider fell in another spiders web and had a bad time crawling......
gpt
We're not *entirely* sure. There is some evidence that spiders make special sticky proteins with a particular anti-stick protein that only they know how to make, which they coat their feet and legs with. That way, they won't stick to their own web. There's also a lot of evidence that spiders build mental maps, or otherwise can tell which parts of the web are sticky and which aren't. Most of the web isn't, actually. They have a lot of support lines that only hold the web up, and even the sticky strings usually have little globs of goo placed along the line, so if you step carefully and know where to step, you can avoid getting stuck. A spider in a strange web would not know where to step, and frankly would probably be too panicked to step carefully enough. In any case, falling into a web doesn't give you a lot of opportunity to carefully avoid getting stuck: a spider would likely get stuck even in their own web if they fell into it.

Unless they can make the anti-stick protein, in which case they would quickly unstick themselves. 

Also keep in mind that not all spiders create web traps like that. Some go out hunting, and although they still use sticky silk to hold onto prey and to descend from high places and such, they don't build the big webs you're used to seeing. Those spiders would be poorly equipped to deal with falling into a web.