Why it's 'un ouvre-boîte': verb plus noun compounds are masculine
French often names everyday objects by joining a verb to a noun: ouvre-boîte (opens + can) is a can opener, sèche-cheveux (dries + hair) is a hair dryer, and lave-vaisselle (washes + dishes) is a dishwasher. These compounds are a quick, logical way to say what a thing does.
The catch is gender. You probably know that la boîte, la vaisselle and la clé are feminine, so it feels natural to say une ouvre-boîte or une porte-clés. But almost all of these compounds are masculine, whatever the gender of the noun inside them: un ouvre-boîte, un lave-vaisselle, un porte-clés.
The first part is a verb in the form you would use with il: ouvre, lave, porte, tire. The second part is the thing the verb acts on. So a tire-bouchon is literally a "pulls-cork" and a pare-brise is a "shields-wind". English does something similar with words like can opener or hair dryer, but in the opposite order: noun first, then the doer.
Here is the key point. The compound as a whole is a new word, and its gender does not come from the noun inside it. It is masculine by default. Think of an invisible un that belongs to the whole gadget, not to the boîte or the bouteille.
Notice that everything agreeing with the compound is masculine too: notre lave-vaisselle est tombé, not tombée.
The kitchen is full of them: lave-vaisselle, lave-linge, grille-pain, ouvre-boîte, ouvre-bouteille, tire-bouchon, épluche-légumes, casse-noisette and the garde-manger (pantry). A quick bite is a casse-croûte.
In the bathroom you find the sèche-cheveux, the essuie-mains, the porte-serviette and the rince-bouche. In the hall there is a portemanteau. On the desk, a taille-crayon, a marque-page and, on screen, the presse-papiers. You carry a porte-clés, a portefeuille, a porte-documents and a passeport, and maybe a couvre-chef. Other useful objects are the réveille-matin, the tournevis, the cure-dent and the lance-pierre.
A car has a pare-brise, an essuie-glace and a pare-chocs. Above the city rises the gratte-ciel.
The same pattern names some ideas and activities, and they are masculine too: passe-temps, casse-tête, couvre-feu, faire-part, crève-cœur and the virelangue, or tongue twister.
The typical mistake is letting the inner noun decide: une porte-clés because of la clé, or la pare-brise because of la brise. Ignore the inner noun and say un.
There are a few exceptions. Garde-robe (wardrobe) is feminine: une garde-robe. Compounds that name a person, such as porte-parole (spokesperson), take un or une depending on who you mean.
Some compounds are written as one word: portefeuille, portemanteau, passeport, tournevis. They follow the same rule, and so do porte-monnaie, abat-jour, porte-avions and essuie-tout.
Plurals vary. Many compounds already end in -s and do not change: un sèche-cheveux, des sèche-cheveux. Others traditionally stay invariable, as in des gratte-ciel, although modern spelling also accepts des gratte-ciels and des ouvre-boîtes. The verb part never takes an -s.
When a French noun is built from a verb plus a noun, like ouvre-boîte, lave-vaisselle or porte-clés, treat it as masculine, even if the noun inside is feminine. Say un ouvre-boîte, le pare-brise, mon porte-clés. The main exception to remember is une garde-robe.