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Throwing one chip in indicates a call unless you say "Raise."
The most obvious example is when playing $1/$2 NL games, you're generally playing with the red $5 chips (because 5 to 10 stacks of white would just be ridiculous). You'll have some white chips in your stack, but mostly red. So when you want to limp, just throw in a red chip. You don't have to say "call" so the dealer knows you're only throwing in $2.
But while most players get that this applies to preflop limping, they don't necessarily get that this applies to post-flop action as well when they're holding green $25 or black $100 chips. In $1/$2, you'll almost never see the black chips, but you can easily see green chips, depending on how much money is on the table.
I'm posting this, well, because most people here don't play a lot of live, and it's good to know the little rules like this.
So anyway, this comes from when I was playing last night, player a had made a post-flop bet of $10 with two red chips. Player b throws in a green chip and doesn't say anything, which I thought was peculiar, because he had plenty of red chips in front of him. When the dealer makes player B change with three red chips, player B gets confused. When it is explained to him that he didn't raise, rather just called (and there was a flush draw on board that hit on the turn), Player B gets very mad and kind of tilty.
Now, as a good player, you generally shouldn't let things tilt you, but...just know that if you're playing live and you throw one chip in, it's a call unless you say raise.