[ QUOTE ]
Roberts Rules of Poker --

Section 3 -- Betting and Raising (Rule 15).

"15. If you put a single chip in the pot that is larger than the bet , but do not announce a raise , you are assumed to have only called. Example: In a $3-$6 game, when a player bets $6 and the next player puts a $25 chip in the pot without saying anything, that player has merely called the $6 bet ."

[/ QUOTE ]

That's the limit rule. The rule is different in No-Limit:

Chapter 14:

11. A bet of a single chip or bill without comment is considered to be the full amount of the chip or bill allowed. However, a player acting on a previous bet with a larger denomination chip or bill is calling the previous bet unless this player makes a verbal declaration to raise the pot. (This includes acting on the forced bet of the big blind .)