There are synomyns, two different words with the same meaning. Antonyms are words that means the opposite of each other. Contronyms, however, are words that are their own opposite.
For example, an alarm can go off, meaning it’s on. Or an alarm can be off, meaning it’s not on. Either way the alarm is off.
To have your student play with this cool part of English, this site explains 55 contronyms. I think it’s fun and essential to play with language if we’re going to appreciate it, use it, and develop it, and a site like this provides definite fodder for playing.