1. Nobody knows what is inside people's heads/head
2. We have green hats on our head/heads
3. They shouldn't go there. There is so cold that their nose/noeses will come off. (everyone has one nose)
4. Just for a laugh. They will be able to stick a sticker on their head/heads
5. The hostages were tied so that they couldn't move their hands. (we have two hands :))
6. The hostages were tied so that they couldn't move their body/bodies.

Could anybody highlight the correct noun, please?
I am confused about using plural or singular noun. I don't understand.

If you could leave a comment clarifying, I will be happy.

thanks in advance