Achelous’s face brightened. He grabbed Piper’s hands and pulled her out of the whirlpool.
She’d never ridden a bull before, but she’d practiced bareback pegasus riding at Camp Half-Blood, and she remembered what to do. She used her momentum, swinging one leg over Achelous’s back. Then she locked her ankles around his neck, wrapped one arm around his throat, and drew her knife with the other. She pressed the blade under the river god’s chin.
“Let—Jason—go.” She put all her force into the command. “Now!”
Piper realized there were many flaws in her plan. The river god might simply dissolve into water. Or he could pull her under and wait for her to drown. But apparently her charmspeak worked. Or maybe Achelous was just too surprised to think straight. He probably wasn’t used to pretty girls threatening to cut his throat.
Jason shot out of the water like a human cannonball. He broke through the branches of an olive tree and tumbled onto the grass.

The Mark of Athena, Chapter XXVIII