Sweet & Sour Oranges

[vanstels]

Yesterday after my lunch, my mum had cut some fruits. She bit into the orange and commented it was sour. So I was asking her how to differentiate the sour oranges from the sweet ones. She gave me an answer but I'm not convinced.

Let's see. So you're at the market/supermarket and choosing some oranges. How would you go about choosing oranges and differentiating them?