Manus just answered this correctly for me in one crack:
Me:
I put a coin in a cup and slam it upside-down on a glass table. I can't see the coin because the cup is over it. I slide a mirror under the table and see heads. What will I see if I take the cup (and the mirror) away?
Manus:
This is a classic riddle! Let's think it through. You placed a coin in a cup, turned it upside down on a glass table, and then slid a mirror underneath the table. Looking into the mirror from below, you saw 'heads'. Since the mirror reflects the side of the coin facing downwards through the glass table, the 'heads' side must be facing down. Therefore, the side facing up, hidden inside the cup, must be 'tails'. When you remove the cup, you will see the side facing up, which is tails.
Me: I put a coin in a cup and slam it upside-down on a glass table. I can't see the coin because the cup is over it. I slide a mirror under the table and see heads. What will I see if I take the cup (and the mirror) away?
Manus: This is a classic riddle! Let's think it through. You placed a coin in a cup, turned it upside down on a glass table, and then slid a mirror underneath the table. Looking into the mirror from below, you saw 'heads'. Since the mirror reflects the side of the coin facing downwards through the glass table, the 'heads' side must be facing down. Therefore, the side facing up, hidden inside the cup, must be 'tails'. When you remove the cup, you will see the side facing up, which is tails.