The hardest thing I ever had to explain to any of my children was when my daughter was 5 or 6. She insisted that a series a numbers such as 1234 is not a number, but most definitely must be 'numbers'. I could see her logic, there is more than one number.
We discussed and debated it often over a good six months but she just couldn't grasp it.
Finally my wife explained it in a way that was acceptable to her (a series of letters such as animal is not letters but a word; a series of numbers is a bigger number. A series of numbers needs commas and spaces between them to be numbers).
What seemingly straightforward thing have you had trouble explaining to your children?
By the way, looking at the word 'numbers' - it doesn't look right, somehow. Surely it should be pronounced 'nummers'? :D
my 9yo is curently being taught about puberty at school. She said to me that as her teacher had told them about periods, willies, boobs moods and smelling what could possibly be left to learn in yrs 5 and 6