How many children does it take to open a jar of jam?

Two it would appear.

One to go and get the jar opening device, attach it to the lid, and then stare at it glumly as the jar turns as well.

The second one to say “you need to hold it” and then proceed to do so, as the first turns the lid on its own this time.

I’ll leave it to your imagination to work out which child did which though