"Is zero a real number?" asked Marcus last night. I think in the specific mathematical sense of "real" that it is. If I remember rightly.
Elf disagreed. She thought it was like black and white which are reputed to not be colours. Marcus thought it wasn't a real number because when you add it or subtract it nothing happens.
I pointed out that its very important in telling the difference between 1, 10 and 100. But also, 0 as an amount is very important. If someone says "Before I visit you I need to know how many tigers you have running loose around your house" - well, the answer 0 is very reassuring.
We sat around the table discussing the conundrum that "nothing" is actually something. It was, for our dinner table, quite intellectually bracing.