Gravity is one of the most fundamental forces in the universe, but if you remove it...

Gravity is one of the most fundamental forces in the universe, but if you remove it, you get gravy.

This is a linguistic pun. The joke is that "if you remove it" could mean "if you remove gravity" in this sentence, but it could also mean "if you remove the letters I and T". Removing the I and T from gravity would leave you with the letters of the word Gravy, so the punch line is reasonable, however this joke is funny because the listener would normally assume that "it" refers to gravity, not to the letter I and T.