Largest even number

The class were shown this problem that required them to use their knowledge of off and even numbers:

The students then worked with a partner to work on similar problems on this webpage: https://nrich.maths.org/7431

When making the largest possible even number they were asked to explain how they knew it was even:

Then, as a class, we created rules for this activity;

The activity on the link provided above can be changed so that the aim is to make the largest possible odd number. You could try this at home!