because conduction takes place in objects whose particles are close/touching each other therefore conduction happens in solid as liquid and gas paricles are not close to each other
The main reason for this is gravity. Gravity between objects in our solar system and milky way is strong enough to counteract this expansion. Hope it helps.
im not an expert about these topics but I think it will float for a while until the water soaks into the wood causing the wooden cube to sink. Sorry if it's wrong, just what I think