Read the full text of Titus Andronicus Act 3 Scene 2 with a side-by-side translation HERE.
At the Andronicus house, Titus, Lavinia, and Marcus sit around the table and enjoy a little snack.
Titus announces that he's grateful he has one hand left so he can use it to beat his chest in woe. Poor Lavinia can't even do that. Perhaps, suggests Titus, Lavinia should put a knife in her mouth and stab her chest. That way, when she cries, her tears will have a place to drain into.
Marcus wisely advises Titus not to say such horrible things to his daughter, who might take him seriously and hurt herself. Only Marcus doesn't exactly choose his words well. He says "teach her not thus to lay / Such violent hands upon her tender life" (our emphasis).
Titus flips out over Marcus's unintended "hand" pun and screams that Lavinia doesn't have any way to "lay such violent hands upon" herself because she doesn't have any hands.
Young Lucius (a.k.a. "Boy" in some editions of the play) enters crying and asks his grandfather to stop being so mean to everyone.
Marcus swats a fly with his knife and Titus goes nuts, asking what if "that fly had a father and mother"?
Marcus smoothes things over by suggesting that it was a "black, ill-favo'd fly" like the "Empress's Moor," which is why Marcus had to kill him.
Titus thinks this explanation is totally logical and proceeds to join in on the fly-stabbing fun.
Marcus says he feels sorry for Titus because the old guy has obviously lost his marbles.
Now that the fly is dead, Titus, Lavinia, and Young Lucius go off to read a book together.