Over the years, we’ve built up what we like to think of as a mental encyclopedia of beauty tips. 

From learning which routine changes can help boost our hair, skin and nails to seeking out the hottest products on Pinterest and taking beauty advice from stars such as Miranda Kerr and Beyonce, we thought we had our beauty knowledge nailed (sorry).

Until now.

Celebrity hairdresser Sam McKnight, who tends to the locks of A-listers including Kate Moss and Madonna, has revealed his number one beauty tip for hair – and it’s pretty unusual.

In an interview with Fantastic Man, the hair genius insisted that we should all be washing our hair with mineral water because it’s “crucial for washing out the hard water” and can help to “balance the pH of your scalp”.

McKnight recommends you rinse your hair with mineral water after shampooing to counteract the "awful” effects of using hard water on your tresses.

Speaking to The Guardian to elaborate on this theory, he said, “Hard water contains higher levels of calcium and magnesium, which can bond to the hair”, which in turn makes it appear both brittle and dull.

Read more: 12 of the best non-toxic beauty buys on the high street, from make-up to hair care

“It can affect how hair absorbs and responds to products, so could potentially prevent your treatments from working as best as they could,” he added.

The advice is the latest in a string of hair washing how-tos from big name hairdressers.

Last month, Christophe Robin, who counts Catherine Deneuve and Tilda Swinton as clients, told the New York Times that we shouldn’t be washing our hair any more than twice a week (which, if we’re using bottled water, at least makes financial sense).

And Sydney-based hairdresser Janet Nguyen, who has a waiting list of over 100 clients, recently shared her top hair styling tips, including a double pony hack and a clever use for a toothbrush.

And if washing your hair with mineral water sounds a little too high maintenance for your daily routine, fear not – here we have compiled our ultimate guide to the golden rules of hair care, and there isn’t a bottle of Evian in sight.

Images: iStock