Due to the effect of heat on water on the molecular level – cold and warm water have different densities. Cold water is more dense than hot water, because without heat, the water molecules can not fuel fast movement.
Cold water molecules are more sluggish, so that when they do bump into one another they slowly pull back and don’t go in opposite directions.
Therefore, the molecules aren’t forced apart by their movement and are packed together more tightly, hence, remaining dense.
GUESS IYKE’s COMMENT: Hmmm! No knowledge is lost