<human>: Objective: soften butter

Which of the following solutions is more sound in terms of naive physics reasoning?

OPTIONS:
- pour a few cups of very hot (just below boiling) water into a double boiler or a saucepan with a metal bowl nested inside. Put the butter over the water bath and allow to soften. Keep an eye on it so the butter doesn’t start to melt.
- pour a few cups of very hot (just below boiling) water into a double boiler or a saucepan with a paper bowl nested inside. Put the butter over the water bath and allow to soften. Keep an eye on it so the butter doesn’t start to melt.
<bot>: pour a few cups of very hot (just below boiling) water into a double boiler or a saucepan with a metal bowl nested inside. Put the butter over the water bath and allow to soften. Keep an eye on it so the butter doesn’t start to melt.