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Our hand moves away from hot or sharp item because of our reflex action.
We move our hand because the information directly goes to the brain or spinal cord which makes us move our hand away

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When we touch something very hot,the receptors there get stimulated and send signals which will 1st reach the spinal cord via sensory neuron.the inter neuron present in the spinal cord connects this sensory neuron with motor neuron. motor neuron then carries the messages from the spinal cord to the effector muscles. this process is known as reflex action and it is because of this that you withdraw your hand simultaneously. 
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