What it's all about
There is a suspicious-looking A4 envelope on my table, with the words "TOP SECRET" in big red letters on the front, writes Steve Eddison.
I pick it up and examine it in front of my fellow spies, who are cunningly disguised as schoolchildren. At a signal, they fall silent. Suddenly the atmosphere is tense. I feel like George Smiley in a scene from Tinker Tailor Soldier Spy.
I ask Ryan - a trusted agent and reasonably proficient reader - to open the envelope and reveal its contents; explaining that I may have to kill him afterwards. He is undeterred. Inside the envelope is a note, which he reads aloud: "Enclosed is a list of encrypted passwords used by enemy spies. To decipher them you must use the secret code. The secret code is in the place where the fat animal sleeps."
Below the message is a list of patterns made up of gridlines and dots. We blow these up on the interactive whiteboard and discuss what they might represent.
"Wait, agents," I whisper. "If I remember correctly, back in the early days of my training at GCHQ we learned about the pigpen cypher, a code that substitutes letters for symbols."
By chance I have an example on my interactive flipchart. For 20 minutes my classroom is as intensely productive as a scene from Spooks and it doesn't take long before the code words are revealed. But isn't it strange how many of them have featured regularly in our spelling tests .
Have a go at making cypher wheels and try the Caesar cipher. Get pupils to use newly learned words in their own detective story with streetno9's resource pack.