The Thursday Packet (two printable pages). Page one is the note: NVVG NV YVHRWV GSV TIVVM WLLI. Page two is a strip of the alphabet, A to Z, with a dotted fold line down the middle, and a tally grid for counting letters.

Try it before chapter 5. Count the letters in the note. Which comes up most often? Then fold the strip so A sits on Z, and read each letter's twin.

The answer. MEET ME BESIDE THE GREEN DOOR. V appears eight times and is E. The code is Atbash: each letter swaps with the one the same distance from the other end.

No printer? Write the alphabet on a strip of paper and fold it by hand.

The grown-up layer. Two ideas meet here. Letter counting is how codes were broken for centuries, because some letters are much more common than others. Atbash is one of the oldest known ciphers. Trying a tool, seeing that it gives nonsense and putting it down is a skill in its own right. HuntForge and CipherForge play with both.

For the ride home. Ask: how did the company know the slide was the wrong tool?