Grocery shopping is always fun and is the only occasion when impulse purchase establishes a strong connection with me. But can you imagine what would happen when you go to a supermarket with a Spanish-speaking flatmate? It was a 110% funny experience.
My new flatmate is from Austria. She speaks perfect Spanish as she lived in Mexico City for two years and is now staying in Barcelona to do a Masters degree. One day, we went to one of the supermarkets together (that was her third day in BCN). When I was happily creating various dishes in my head while walking aisle to aisle looking for the ingredients, she asked me: “¿Sabes dónde están aceitunas?” (Do you know where olives are?)
Good question! We were right at the aisle between the one where bottles of olive oil (and all kinds of oil) are and the one where tinned tuna (and all types of tinned food) is. I showed her where the Aisle of Oil (aceite) and the Aisle of Tinned Tuna (atún) were.
What happened next was very obvious- embarrassment, giggles, and strings of English-Spanish bilingual words coming out from both of our vocal cords. Let’s focus on the reason why, shall we? :-P
Aceituna (n.) Fruit of olive tree, i.e. olives (green/black). I love green olives. (Me encantan aceitunas verdes.)
Aceite (n.) Oily liquid obtained from fruits or seeds. We usually use olive oil when we are cooking in Spain. (En España, siempre usamos aceite de oliva para cocinar.)
Atún (n.) Teleost fish, common in Spanish seas. Tinned tuna is very cheap. (El atún en conserva es muy barato.)
Definitions (translated): Real Academia Española
In short:

Ps- Tuna (n.) does exist in Spanish, however, its literal meaning is “a group of university students in traditional university dress who play traditional instruments and sing serenades” (from Wikipedia)