Why can't you flick a tick off you or your pet's skin? The answer is in the tick's mouth that's covered with hooks evolved so the tick can hang on for a several day feast of delicious blood. From KQED's Deep Look:
…A tick digs in using two sets of hooks. Each set looks like a hand with three hooked fingers. The hooks dig in and wriggle into the skin. Then these "hands" bend in unison to perform approximately half-a-dozen breaststrokes that pull skin out of the way so the tick can push in a long stubby part called the hypostome.
"It's almost like swimming into the skin," said Dania Richter, a biologist at the Technische Universität Braunschweig in Germany, who has studied the mechanism closely. "By bending the hooks it's engaging the skin. It's pulling the skin when it retracts."