Using timelines to add and subtract…

Me: “So let’s figure out what year your family members were born. How old is Mom?”
Kid: “30.”
Me (surprised): “Oh, ok. So let’s use the timeline to jump back by 10s and figure out the year she was born.”
Kid: “1984.”
Me: “Yes! So how old was mom in, say, 2010?”
Kid: “30.”
Me: “Hmmm. Are you sure?”
Kid: “Yes. She’s always 30.”