Pick up the phone

Marc's mother was on the phone to him and at the end of the call she asked to speak to the kids. Marc called downstairs and asked Shira to pick up the phone. After a minute, when she wasn't on the line, he reminded her to pick up the phone.

"I have picked it up," she responded. Marc and I collapsed laughing. "Now answer it!", we chocked out over our laughter. We are forever being caught by how literal our children are.

When do children stop being so literal?