Bad news

So, how exactly do you tell someone bad news when you at least seem to be happy? I don't want to ruin anyone's mood either, but meh. Hiding isn't the right thing to do either, right? Okay this sucks. You can't give someone bad news while you seem to be happy and when you know you will ruin the person's mood. But if you hide it and that person finds out, it'll still hurt. I hate it when I have to decide such things. Especially when I already know I can't talk about it more then. T_T