What begins with knock and ends with knock?

Knock knock.

“Who could that be at this hour?” thought Julian.

He decided to ignore it, but the knocking persisted, and finally, cursing softly, he rose from his saline bath, small rivulets of serum coursing down his iridescent green tentacles.

He opened his futuristic space door, revealing a large gleaming metallic orb floating a metre above his welcome mat.

“Uh, hello, can I help…” Before he could finish his sentence, a sharp metal spike shot out of the orb and pierced Julian through his sclernix*.

“What… the… fuck….” Julian sputtered weakly, sinking to the floor. He shuddered once, and died. Purple blood welled from his sclernix, silently filling the grooves between the tiles of the foyer floor.

In a flat synthetic voice, the orb said, “Orange you glad I didn’t say ‘Firetruck’?”

*sclernix: the vulnerable primary sensing organ of the extraterrestrial species invented for the purposes of this antiantijoke.