Great for phonics or numicon - basically any topic where you want to put a clue on the outside and answer inside. Could even put different textures/colours or smells inside to discover in a sensory area.
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Fine motor and maths activity. Children have to do the sum, or, for younger children, count the dots, and then match the key up to the correct padlock to open it. This activity not only improves their maths, but also strengthens the muscles in their hands and fingers, which is important for improving writing skills.