I just explained to my 12-year-old nephew what an RPG is, at a dinner party, in five minutes.
Me: You are climbing up spiral stairs inside a turret, looking for a kidnapped princess. The stairs end at a door. What do you do?
Nephew: Huh?... well...I open it.
Me: Push or pull?
Nephew: Pull!
Me: Too bad, it's for pushing.
Nephew: Okay, then I push.
Me: It opens onto a circular room, with a narrow window on the other side. Next to it you can see the princess, her ankle chained to a wall. But between you two are three nasty-looking soldiers, brandishing swords, and they're coming at you! What do you do?
Nephew: Er... do I have a sword?
Me: Yes, a sword, a shield and a nice armor.
Nephew: Then I go at them!
Me: Ok, let's flip a coin. If it's head, you hit the bad guy. If it's tails, he hits you, okay?
(flip, flip, flip, I let him vanquish all three brigands)
Me: Okay, the princess is still chained to a wall.
Nephew: I hit the chain with my sword to free her.
Me: Okay, you cut open the chain, and you two start running down the stairs. When you get to the courtyard, you can see, to one side, the open gate, and your horse next to it. On the other side, a band of brigands are coming, screaming curses and brandishing swords. What do you do?
Nephew: I run for my horse, put the princess on it, then I look for some button (heh) to drop the gate!
Me: A lever, not a button... Okay, you find one, flip it and run to the horse while the gate closes and traps the ruffians inside the castle. You mount your horse along with the princess and ride back home.
Me: Congratulations, you've just played your first ten minutes of RPG.
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As for the actual DnD, it's best to ask each player with some advance which type of character he/she'd want to play. I usually describe each class very broadly, and very archetypal. As the game goes along, you describe each roll as they occur. "I want to attack him!" "Okay, you just roll that d20 and add your attack modifiers, written... here... in your sheet" "Okay... uh... 16?" "okay, you hit him. Now roll... this die, a d8, and add your Strength modifier, written... here." "Okay. Wow! 9!" "Cool! You run him through with your sword, and as you pull it out, he fall to the floor with a 'clunk'." "Cool!"
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