There is a foodie truism (passed down to me by my mum) that when abroad you should eat where the locals eat. I am a very obedient son.
I have eaten ceviche in a truckers' cafe in Lima - roughly chopped and astringent, and served alongside a soft, steaming, deep-orange nub of sweet potato. On my honeymoon, I dragged my new wife into a crumbling, strip-lit roadside cafe where we ate exquisite vegetable thalis off banana leaves served for under a dollar. I know how to treat a lady.
The rule only failed me once: when I ordered the ducks'-blood soup in a neighbourhood restaurant in Laos. It turns out this dish is popular with the locals for reasons of machismo rather than flavour. Despite looking beautiful - bright red with golden crispy shallots on top - it tasted of what it was: cold, coagulating blood. Little chewy clots had started to form in it. I choked it down to prove my manhood, but the memory still makes me wince.
This week's recipe, watermelon agua fresca (literally "fresh water") has altogether happier memories. A leathery old man used to serve it from a trolley on a street corner near the school in Oaxaca (pronounced Wahaca) in Mexico where I went to learn Spanish many years ago. It was cold, refreshing, a beautiful colour and very slightly sweet. (You can leave out the honey if you like).
Tip: When buying watermelon juice from a streetside vendor, be sure you can trust the source. Watermelon is one of the few fruits that doesn't filter bacteria out of the water that it draws up from the ground.
Make your own watermelon agua fresca
1 tbsp honey
Juice of 1 lime
A pinch of salt
500ml soda water
Toasted cumin seeds (optional)
1 Blend the watermelon with the honey, lime, salt and water.
2 Strain through a coarse sieve, pushing the watermelon pulp through. A mouli-legumes or veg mill can be used to do this step. Refrigerate for at least 2 hours.
3 To serve, divide the mix between tall glasses filled with ice and top with soda water. The drinks can be finished off with sprigs of mint, lime wedges and a sprinkle of toasted cumin seeds.
Think pink: a refreshing watermelon soda. Photograph: Jill Mead for the Guardian