This in itself will provoke discussion amongst pupils and you may wish to address the misconception that tilted squares are called "diamonds". In order to find the largest number of counters which can be placed on the grid, children will have to work in a very systematic way. You might like to ask the class to have a go at the problem to begin with and then to stop them after a short period of time to find out how they are tackling it. This might be an opportunity to model a systematic approach, for example by placing counters on the grid from top left to bottom right and each time checking that a square has not been made.