'Bijou', 'genou', 'chou' and the seven '-ou' nouns that add '-x' in the plural
Most French nouns ending in -ou form their plural the normal way, with -s: one trou, two trous. A small, closed club breaks this rule and takes -x instead: bijoux, cailloux, choux, genoux, hiboux, joujoux and poux.
English speakers trip over this for two reasons. First, the -x is silent, so genou and genoux sound exactly the same and your ear gives you no clue. Second, some of these words are very common in daily life (knees, jewellery, cabbage), so the mistake shows up often in writing. The good news is that the list is short and fixed. Learn it once and you are done.
The default rule for -ou nouns is simply to add -s. Only seven nouns take -x: bijou (jewel), caillou (pebble), chou (cabbage), genou (knee), hibou (owl), joujou (a childish word for toy) and pou (louse). Every other -ou noun you meet, such as bisou, cou, trou, clou or kangourou, takes a regular -s.
French children learn the seven with a silly rhyme you can borrow: Viens, mon chou, mon bijou, mon joujou, sur mes genoux, et jette des cailloux à ce hibou plein de poux ! (Come, my darling, my jewel, my toy, onto my lap, and throw pebbles at this owl full of lice!) If a word is not in the rhyme, it takes -s.
Remember that the -x is only a spelling. You hear the plural in the article (le becomes les, un becomes des), not in the noun itself.
Genou is the most useful member at A2 level. In the plural, genoux also means lap, as in sitting on someone's knees. In the expression avoir mal aux genoux, the à + les contracts to aux, so you get two x in a row.
A single bijou is one piece of jewellery. English uses the uncountable word jewellery, but French usually says des bijoux or les bijoux, always with -x.
Chou keeps its -x in compounds too: choux de Bruxelles, choux-fleurs, choux-raves. With the hyphenated compounds, both parts take a plural ending.
These are less frequent but just as fixed. Hibou starts with an aspirated h, so there is no liaison: les hiboux is said without a z sound.
The typical mistake goes both ways. Learners who remember that some -ou nouns take -x start adding it everywhere and write trouX or bisouX. Those forms are wrong. Words like bisou, cou, trou, clou and kangourou are regular.
Also note that mon chou and mon bijou are affectionate names, like darling or sweetheart. They follow the same plural: mes choux.
Add -s to nouns in -ou, except for the seven in the rhyme: bijou, caillou, chou, genou, hibou, joujou and pou, which take a silent -x. Compounds of chou keep the -x, and everything else, from bisous to kangourous, is regular.