How do you know what hashtags to use?

[lisa]

I understand the whole concept of Hashtags, but how do you know which ones to use, I mean..how do you know what hashtags are out there? Do people know them off the top of their heads or from being online so much, so can you type in the first letter and a list will show up or what?
Jessica, if you can't answer in a tasteful manner, you should not be on Yahoo answers. We ask questions we don't know the answer to, and just because you know the answer, doesn't mean others are stupid, everyone is here to help each other, not bring each other down! I'm sorry you are so rude, sweetie, and I hope you get better soon! <3

And just to clarify: I meant the longer hash tags (not one word!) for example #awesomestthingontheenternet, that kind of hash tag, how would I even have known that existed?

That is what I meant, my dear Jessica.

[Jessica]

are you stupid?
if it's relevant to the photo, tag it. chances are someone will look that tag up and see whatever it is you tagged.
it's not that fucking hard.
if it's a photo of the beach, you tag it #beach.

wrong category btw.