Mary: "Hi! Where's your mother?"
Mrs. Daughter: "Oh, she'll be here. She had a lot of errands to run, and won't get here until 10:30."
Mary: "Her appointment is now, not 10:30."
Mrs. Daughter: "I know that. It's why I came in on time. I'm holding the slot for her."
Mary: "Excuse me?"
Mrs. Daughter: "I'm here, since she can't be. I'm holding the slot, so you won't cancel it before she gets here. She'll be here at 10:30, I promise, and in the meantime I'll wait here in her place."
Mary: "But you're not the patient!"
Mrs. Daughter: "What does that have to do with it? I'm here on her behalf."