The piece under key hinge –– that looks like a nipple –– came loose. What to do? The left arrow key on my 2014 model Mac Air has been acting weird, so I took off the key cap and hinge to see what was going on and I discovered that the tiny piece that looks like a nipple (or a suction cup, if you will) is not attached to anything. I took off another small key to see how it should be. The nipples under other keys are firmly attached to either side of the key hole, but this one is kinda loose. 

As a result, the key works but you have to push really hard on it.

How can I fix this? Can I just superglue it in place or do I need to get a new nipple-thingy?

thanks.
The piece is the "dome" - it sits under the key cap (see here). Underside of it has a conductive surface, so pushing it down closes the contact to trigger a key press. You may be able to fix this with some very small bits of double-sided adhesive tape; not sure I'd superglue it, because failure to fix could mean you're looking at a total keyboard replacement, depending on where the glue gets.