to say or do something that shows someone that their beliefs are false, or that what they want to happen will not happen (I'm sorry to burst your bubble, but... we weren't)
to be quick to understand and react to things (I din’t sleep well last night and I’m not really on the ball today); indicating competence, alertness or intelligence (a women like that, with so much on the ball)
If you do something at the drop of a hat, you do it immediately without stopping to think about it (People will file lawsuits at the drop of a hat these days)