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”.

Celebrity hairdresser Sam McKnight has a very controversial tip for washing your locks