Listen up, people.
A tic is a small involuntary or habitual motion: Only the nervous tic in his left eyelid betrayed his agitation.
A tick is a bloodsucking arachnid: After his walk in the woods, he found a deer tick just above the top edge of his right sock.
They mean two different things, and they have two different spellings.
Got it?
Good.
Dr. Doyle's Blog 
The nervous tick in his left eyelid raised his mandibles and then bit down. Ah, he thought, that’s some good humor at last, the aqueous kind.