New puzzler time again. 

Here we go. 

Several years ago, many many years ago actually, we discovered a nifty way for detecting blown cylinder head gaskets. We used to use a dye that would change color when you remove the vapors out of the cooling system through the radiator cap. When you take the radiator cap off, we discovered that we could pick up those same vapors with our emissions tester, and in fact check for the presence of hydrocarbons in the coolant.

So you take the radiator cap off and get the engine up to operating temperature and you'd bring the tester over and place it near the radiator and if the thing didn't read zero, you knew you had a problem. 

The reason this works is that when you have a blown head gasket, the products of combustion are going right from the combustion chamber into the cooling system, and pretty soon after that they get to the radiator and then come out. And at that point, the machine that you use to test it can smell when this happens. We can't smell it, but the machine can. 

So one time, one of our mechanics was doing this test on a car and he turns to another mechanic and he says, "This guy's in trouble. He's got a blown head gasket."

Another mechanic doing a tune up nearby hears them talking. He hears them say, "I can see he's got like 60 or 70 parts per million of hydrocarbons when I do the test at the radiator here."

And without even lifting his head, the third mechanic says to the first two, "I don't think so..." 

So the puzzler is, what did the third mechanic know that allowed him to say, "I don't think so..." with such conviction? 

Good luck.