Pretend to give them your undivided attention when they are holding forth passionately on something intellectual, like "existential Zen in post-revolutionary Iran," and then suddenly interrupt with one of the following or a cousin thereof:

1. "Oh, hold on a minute, the baby's crying/I need to put the clothes in the dryer"
2. "You've got some ketchup on your nose"
3. Wait for a cat/child to come up to you and then grab for it saying, "come here kitty/sweetie!"
4. Take an obviously irrelevant phone call and make no attempt to close it early and get back to the conversation.

Enjoy the deer-in-headlights look.

More aggressive versions include "say it, don't spray it" and perhaps the best of all: "you're really passionate about this, aren't you?"

If you're at the "intelligent" receiving end of the conversation (scare quotes because if you are that smart, why are you getting gamed like this?), the best response is always to either say "you're not really interested, are you?" and shut up, but NEVER pout. If they protest "No, no, I want to hear this!" you say, "well, you can look it all up in Wikipedia anyway." If they insist, say, "all right, I'll give you the simplified version,..." and produce a totally patronizing version that insults their intelligence.

DO NOT visibly take offense. If their behavior is intentional ("you are not worth full attention/I am indulging you like a precocious kid") the response is designed to use the same attitude towards them ("you aren't worth the full-attention explanation/I am indulging a curious but not very bright kid")