'A closed thermos with 1 liter of water has a higher mass when the water is 80 °C than when it is 20 °C. The difference in internal energy (movement and potential energy of the water molecules) is 2.5 × 105 J, so the difference in mass is 2.8 × 10−12 kg. This is difficult or impossible to measure. If the thermos allows the heat to leak away, it therefore loses some internal energy and a little mass (here too, of course, it is not said that "mass is converted into energy").' https://nl. wikipedia. org/wiki/Massa-energierelatie#Voorb eelden
This is an example of E=m. c² from Wikipedia but I do not understand it.
<=> m=c²/E
So more energy would mean less mass but how do you get more mass if the watermolecules have more more energy (bc of the heath)??