shivverNow here's a challenge. Picture yourself picking up a novel and opening to a random page and starting to read whatever's there. Two (or more characters) are talking (maybe they're just sitting and talking, or maybe they're in the middle of trying to save the situation and are shouting at each other to do things, or maybe it's something else entirely). You've come in right in the middle, so you have no context, and they're not overtly stating what's going on, but you get an idea of what's going on based on what they're saying.
That's the challenge: write the middle of a scene, giving no background or context to us, the readers of this comm, but we should be able infer a bit from the dialogue. This might be easiest to do if you work on a scene of a longer WIP - write a small bit of a scene you haven't gotten to yet.
Remember, you want the dialogue to feel natural, so look at it from the point of view of the characters - what would real people actually say? - rather than the point of view of the reader or writer. For example, if your two characters are in the middle of a conversation about someone else, they may never actually say that person's name during the challenge; they may only refer to them as he or she.