Assumptions.

Two closed containers (air/water tight) each with the EXACT same amount of water.

Container 1:
Is Frozen with the water inside

Container 2:
Is heated to boil the water inside

At the precise moment, whether the container was just frozen or boiled, it is weighed.

Please tell me, which one is heavier? And Please explain to me why you say that.

Have fun!

---Roman