Concept: Children's furniture can weaken when the wood changes size after drying.Explanation:Timber naturally absorbs or loses moisture, causing it to shrink or swell.When timber shrinks, joints become loose and furniture becomes weak and wobbly.Painting or polishing affects appearance and protection, not the main cause of structural weakness.A "broken tongue" is a specific joint failure, not the main reason for overall weakness.Answer: D. shrinkage of timber used.