35/65 Rule

The 35/65 rule states that any two different hole cards in Hold'em will pair the flop 35% of the time. For instance, if you are holding 10,8 a 10 or 8 will come on the flop about 35% of the time. The other 65% of the time you will miss the flop. Aggressive opponents know that if they make a continuation bet on a majority of flops you will be unable to call 65% of the time -- and maybe more if you are willing to fold bottom pair.