In Esperanto if you know the word for "young", for example, you don't have to learn another word to say "old" you can make the opposite by adding "mal" in front of the word.
Try to match the words under the line, just drag the word that you think means "small" into the rectangle, then do the same for the other words. If you chose the wrong word, the computer will not let you drop it into the box.
Note for parents. This exercise illustrates one of the many features that exist in Esperanto to make learning a language a task that is not too time consuming. In most languages you need to memorise 2 words like good/bad. In Esperanto if you know the word "good" you don't have to learn a word for "bad" as "bad" is the opposite of "good".