The Xup Xup

Just as in English, Spanish is full of onomatopoeia. While birds go pio pio, an owl is callen a buho and light rain chiri-miri (cheeree-meeree), my favourite Spanish onomatopoeia is chup-chup (choop-choop). In all honesty I like the Catalan word, xup-xup (shoop-shoop) better but that is because I like the idea of an 'x' being pronounced as a 'sh'
To get an idea what chup-chup actually means go and stand beside a simmering pot and listen. You'll hear the gentle chup-chup, or xup-xup as the bubbles of liquid rise and fall. Too low a heat and no chup-chup, too high and the furious boiling destroys the more gentle sound.
When a Spanish recipe says, "let it go chup-chup for fifteen minutes" every Spaniard reading knows what that means and there is no need for any further instruction on levels of heat. They just stand by the pot and listen.