The normal rules for I Spy apply. The teacher starts with "I spy, with my little eye, something..." and then describes the object by giving hints in English. For younger students, I usually pick something very easy like a chair or textbook. For older students, I pick something harder to describe. Once my students have played a bit and got the hang of it, they take turns being the one to spy.
The first time I show this to students, I go through this PowerPoint, and then after that, I will leave just the last slide up so they can look at it for ideas. I try to encourage them to go for less obvious descriptions than just color and shape every time. They have a lot of fun.
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