"s"

This seems not a rule, but in English, when the subject is plural persons and an object belongs to each, speakers tend to choose a plural form for the object.
We have changed our minds.

As for the second case, I personally think you can use either.
(1) You (all), please open your books.
(2) You (each), please open your book.

I hope our teachers give you mo

Hello Spring, hello Paco

Let's say I'm having a party. Two of my guests leave together (to my intense chagrin) at 10o/c. An hour later there's a knock at the door. I open it to find my guests (looking a little sheepish) on the doorstep. They probably say: 'we changed our mind'.

But let's say 30 guests left my party separately, at intervals of 5 minutes, beginning at 10o/c. At

Hello Paco and MrP

Thank you Paco and MrP for your explanations.

MrP why it wouldn't surprise you if your sheepish guests said, "We changed our minds."
Which one is correct?

Thank you very much for your help!
Spring

This may be a little difficult if you are a speaker or student of French.
The French say, "All the dogs wagged their tail."
Speakers of English say, "All the dogs wagged their tails."

Personally, I use the plural in all such situations. ("changed our minds", "open your books", etc.)

Nevertheless, in the case of "changed our mind(s)", the singular is acceptable when

Thank you CJ for your explanation.
It was very helpful.

Spring