Whilst drawing this I learned about the reasonably interesting origins of the term "I Spy". According to Phrases.org:
The game I Spy originated in the early 20th century. It remains a common pastime played by children - albeit often initiated by adults to occupy bored children on car journeys and the like. One person secretly choose an object that they can 'spy with their little eye' and the others take turns to guess the name of the object.
The game isn't especially old and the first record of it that I can find is in The Winnipeg Free Press, December 1937:The game spawned a highly successful series of I-SPY spotter's guide books made for British children. These were very popular in the 1950s and 1960s.
"Other games ... are: What is My Thought Like, I Spy With My Little Eye (children love this) and Bird, Beast, Flower or Fish."
The guessing game was preceded by another children's game called I Spy (or Hy Spy). This was a variant of what is now called Hide and Seek and was known in the UK from the 18th century. John Brand refers to it in his
History and antiquities of the town and county of Newcastle-upon-Tyne, 1789:
"'I spye', is the usual exclamation at a childish game called 'Hie, spy, hie'."