Someone explain this to me, related to tire valves
 

So I finally listed my old rims/tires on craigslist. Well I went to inflat them to the appropriate psi and I found out 2 of the tires have the needle missing inside the tire valves. Would anyone care to explain how that happens? BTW you can see cracks at the base of the tire valve where it meets the rim so I'm assuming you'd want to replace them anyway, but how does that happen with the needle missing?

RE: Someone explain this to me, related to tire valves
 

The valve screws into the stem.
Some tire places will unscrew them to deflate/inflate the tire faster.
Sounds like you need to replace the stems anyway.

RE: Someone explain this to me, related to tire valves
 

It's really easy to screw them back in. I bet if you went to autozone they would have the tool as well as the stem for you