My grandma puts a donut in one of two boxes, closes both, gives one to each of us. You leave for Mars and, at an agreed-upon instant, I open my box. Now I know

My grandma puts a donut in one of two boxes, closes both, gives one to each of us. You leave for Mars and, at an agreed-upon instant, I open my box. Now I know what is in your box, and you know what's in mine. No faster-than-light, no mystery.