What part of the onion makes you cry?
When you cut into an onion, you break cells, which release amino acid sulfoxides form sulfenic acids. This burns your eyes and makes you cry. So basically, the whole onion. xD
The part you peel or cut
When you cut an onion, it releases a gas. It's not the smell at all!
The entire thing. When you cut an onion, it releases fumes and acids that make your eyes teary. You aren't actually crying, though. :)