Whenever you find the capital/lowercase pair they will disappear from the screen, and when there are no more letters to find it means you've finished the game.

The game was created by Antonia's idea (from Canal Antonia Desde Pequena - https://www.youtube.com/channel/UCnpo2jI0LzJeguTM20dq35w) to facilitate the learning of lowercase letters, because at the time she was very good at capital letters only. And, with the game she played and ended up learning all the pairs of capital and small letters.