the water is warm & a hot / warm something is a source of heat ->  the warm water is a source of heat  the warm water is a source of heat & a metal lid is put under warm water ->  the metal lid is put under a source of heat  the metal lid is put under a source of heat & if a thermal conductor / an object is exposed to a source of heat then that conductor / that object may become hot / warm ->  the metal lid will become hot under the warm water  the metal lid will become hot under the warm water & if a material is heated , then the material will expand & metal is a kind of material ->  the metal lid will expand under the warm water  the metal lid will expand under the warm water & if the lid of a container expands then that container will be easier to open -> the lid is easier to open because the metal lid expands under the warm water