Cat forgets something;
"Did I leave the kettle on? No, I'm a fucking cat."
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And since my cat used to do this, and someone up thread said their cat did it, too, I wonder if they leave it out on purpose. 
He doesn't leave his tongue hanging out.
Instead, he will sometimes sit upright and groom his paws, and then stop in the middle of licking a paw and just kind of forget about it. So he'll be sitting there staring off into space with one paw upraised for minutes at a time. It's very funny. I always say, "did you forget to put your paw away?", and he looks at me blankly, like "What?"
I always wonder what happens when they do stuff like that. When he forgets to put his paw away, it very much looks like he was grooming and then had some kind of very intense, important thought that caused him to become absent-minded and simply have to pursue it to its conclusion at the expense of all other activity. I know in reality there can't be much between his ears besides fluff, whistling air, and thoughts about cat food. But he still sometimes comes across as a little fuzzy philosopher, or small daydreaming feline professor or something. I love him, and he's a great little funny friend. 
Sometimes in the middle of grooming, she'll stop and look away and I can see her pink tongue just hanging out for a couple of dozen seconds 