GCHQ's experts have created a new collection of brainteasers for the intelligence agency's second puzzle book.
The book also features authentic entrance exams set for would-be recruits and anecdotes about the people, technology and partnerships that are central to GCHQ's mission.
The agency, which works alongside other intelligence services and law enforcement to protect the UK from terrorism and cyber attacks, marks its centenary year in 2019.
But do you have the mental know-how to become Britain's top spy?
Here are some examples:
Q 1. Which is the odd word out?
Q 2. Take a break - explain the order:
Head, Belly, Fingers, Nose, Beard, Eye, Leg
Q 3. Missing animal - fill in the gap:
Snake, Penguin, Dog, Ant, ?, Crab
Did you get any right?
A 1. Nation.
The other words can follow Super Mario in the titles to video games, whereas supermarionation is a style of puppetry popularised by Thunderbirds.
A 2. They are in snooker-ball value order:
(red) Head, (yellow) Belly, (green) Fingers, (brown) Nose, (blue) Beard, (pink) Eye, (black) Leg.
A 3. e.g. Spider.
These are creatures with 0, 2, 4, 6, 8 and 10 legs respectively.
Still want more? Here are some examples which featured in the book:
Or you can have a go at GCHQ's Christmas card with a difference.
How to play
Players must complete a grid-shading cryptographic puzzle with a black pen to reveal a picture.
Full instructions can be found on the agency's website.