It's weird -- you're talking, and your teen appears to be listening. You're so impressed, and before long, you've run out of things to say. But the only thing your teen's paying attention to is an attempt to wind this conversation up as quickly as possible without learning too much in the process.
During "the talk," it's easy for a teen to pipe down (for once) and let you carry the conversational load. Your kid's trying to run out the clock -- if he or she sits there and nods in agreement long enough, at some point this ordeal will be over, freeing up your teen to go hide under a rock and die of embarrassment.
Set a give-and-take tone for the conversation that allows your teens to broach subjects or ask questions they've been wondering about. Listen to them, and make sure you're providing them with a chance to talk and ask questions.
It's important not to concentrate solely on plowing through your prepared PowerPoint presentation. Pay attention to their murmurings or mutterings, because they may provide you with the opportunity to get their actual thoughts on the matter.
Next: Your teen's pretty sure you're not the brightest bulb around.