Of course, they needed to actually make something, so he prepared the parts they needed to make a wooden puzzle - The Tower of Hanoi.
Step 1: What you need.
The boys used 15mm-thick MDF for the base, 5mm thick MDF for the puzzle-pieces and 5mm dowel for the posts.
The tools they used included coping saws, tenon saws, scroll-saws, files, glass-paper, a pedestal drill, power-sander, band-saw, circular saw and a mallet.
Oh, and goggles.
Step 2: The base.
The holes need to be around 50mm apart to allow the puzzle-pieces to sit side-by-side.
Using the pedestal drill and a 5mm bit, drill the holes about three-quarters of the way through the base. If you are able to, set the stop on the drill to prevent it going all the way through.
After drilling each hole, knock the dust off the base, don't blow it off.
You will, of course, need to wear goggles.
Step 3: Shaping the base
They drew outlines on the bases, leaving enough wood around the holes for strength.
They practised using a coping saw and a tenon saw on scrap wood, and decided they would rather use a power tool to cut out their proper bases.
So, roll out the scroll saw. Goggles on again, make sure the guard is placed to allow the base through without finger, and let Grandad hold the base firmly while you guide it through.