Some math riddles tend to be spoiled for mathematicians before they get a chance to solve them. Three examples:

What is $1+2+\cdots+100$?

Is it possible to tile a mutilated chess board with dominoes?

Given a line $\ell$ in the plane and two points $p$ and $q$ on the same side of $\ell$, what is the shortest path from $p$ to $\ell$ to $q$?

I would like to give my children the opportunity to solve these riddles before the spoilers inevitably arrive.

Question:What are other examples of kid-friendly math riddles that are frequently spoiled for mathematicians?

*Notes*:

There is no shortage of kid-friendly math riddles. I am specifically asking for riddles that are frequently spoiled for mathematicians because they capture a bigger idea that is useful in math, especially research-level math. As such, the types of riddles I am asking for are most readily supplied by research mathematicians.

In case it is not clear whether MO is an appropriate forum for this question, see the following noteworthy precedent: Mathematical games interesting to both you and a 5+-year-old child

My Best Mathematical and Logic Puzzles. Not all of these are suitable for kids but some are. Sometimes he also prefaces the statement of a puzzle with an easier "too often spoiled" puzzle (is there more water in the wine or more wine in the water?) so it's also a good source for those. $\endgroup$11more comments