The simplest way is to dismantle a child's toy that has a DC battery-powered motor and recover the motor. When the shaft is rotated by some form of turning force (home-brew windmill rotor out of scraps of balsa wood from the local model shop and some paper plates for use as sails, perhaps?) a voltage will appear at its terminals that can be used to light, oh, say, a torch bulb?
All it takes is some imagination and some experimentation!
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