I did not know the difference between a hare and a rabbit, or a prawn and a shrimp. South Africa always talks about prawns, and Sweden talks about shrimp so I always thought they were different words for more or less the same thing. Or that prawns were bigger than shrimp. Swedish has one word for both tortoise and turtle (sköldpadda).
When my students confuse this, I tend to teach ‘if you bite it and you die – it’s poisonous, if it bites you and you die – it’s venomous.
Source: The Language Nerds