While Penny is getting larger and slowing down, Tilly still wants to run. And although Penny has never been a fan of playing fetch, Tilly will keep bringing back the ball longer than seems prudent, given the chance. If we didn't give Tilly a ball to chase every day, she would try to get her little (though now large with pups) sister to run with her, something we want to avoid.
Henry took these pictures while letting Tilly run around.