Buy first to remove watermark! Great for beginner Spanish students when teaching about regular -o and -a rules. For masculine nouns, I use LONERS (plus -ema, -eta, etc.) and then the usual feminine patterns like -ción, -dad, -tud, etc.)
There is also practice for using definite and indefinite articles at the bottom to teach them to always check:
Do all your words match in gender?
Do all your words match in number?
Does your verb choice match in number?
Are your articles correct?