- The water is warm
- A hot / warm something is a source of heat
- So, the warm water is a source of heat

- The warm water is a source of heat
- A metal lid is put under warm water
- Hence, 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
- Therefore, 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
- So, 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
- So, the lid is easier to open because the metal lid expands under the warm water

So, the lid is easier to open because the metal lid expands under the warm water
[Question]The metal lid on a glass jar is hard to open, so it is held under warm, running water. What causes the jar to open easily after it was held under the water?
[Context]- If a material is heated , then the material will expand 
- The water is warm 
- If a thermal conductor / an object is exposed to a source of heat then that conductor / that object may become hot / warm 
- Metal is a kind of material 
- A metal lid is put under warm water 
- If the lid of a container expands then that container will be easier to open 
- A hot / warm something is a source of heat