A friend said something disturbing to you yesternight. Today evening she came to me and apologized. She says "Can we just start being like what we were before yesterday?".
The question is very innocent. And I don't want to answer it in words. Because a "No" sounds awful.
So I give her a cup of water to drink. She drinks from it, finishes it. Now I ask her "Give me the water back". She is stunned - she got her answer.
Just like a bullet can't re-enter the gun after firing, in the same way you can't take back your words.
So think not twice but thrice before you speak.