A bottle cap resource - Fill in the Missing Numbers

I went to a Jolly Phonics training session a few years ago and there they showed us the idea of using bottle caps to write letters on them so children can make words. This is a great idea simply because they are durable and if lost, very easily replaced. I have since become a little obsessed with bottle caps and people gladly hand them over. I have taken the idea and used bottle caps to make a "Fill in the missing numbers" game.