In S11E20 of The Big Bang Theory, Rajesh compliments Wolcott's tomatoes. He tells him that the tomatoes were fertilized with human manure. Howard and Leonard joke
The look on your face.
It's a sort of grin, wanna know what kind?
Not being a native English-speaker, I'm missing the joke here. I assumed it was an expression, maybe something like this, but I'm not sure. What kind of grin did Raj have?