Why won't she tell me its really annoying?

there is a girl, and i kinda like her, and she says she likes someone put won't tell me who it is. shes quite flirty and all the time, when i ask her who she on likes on msn, she says its me and then says she is joking after a few seconds, and then she goes off saying something like "love ya, bye xxxxxx" it is really annoying lol, what are ur fews on this, anyway thanks for reading.