I've noticed that the cans that were stacked by Zizi in a triangle were $4$ high. Not only that but I counted and found that she used $10$ cans.
Pete had made a triangle just $3$ high and I noticed that he used just $6$.
"Both even numbers!"
On another table Cas made a triangle $5$ high and used an odd number this time, $15$! and Sophie made an even bigger one of $21$ this was $6$ high.
"These were odd numbers!"
What can you can about these pairs of even and then a pair of odds - will it often happen?
What do you think?
Can you prove that your ideas are right to others?