John Mayer will always sing 'Happy Birthday' to strangers.

The 'Gravity' hitmaker learned a valuable lesson in small acts of kindness when he was out for his own birthday one year and, when his pals sang to him, Motown legend Stevie Wonder - who was seated a few tables away - joined in, making the occasion even more special and memorable for the 42-year-old star.

The moment has always stuck with John so, if he finds himself in a situation where he's near to people celebrating their birthdays, he will always join in with the singing.

Writing in honour of Stevie's 70th birthday on Wednesday (13.05.20), he shared on Instagram: ''Happy Birthday Stevie Wonder! I'd like to share a story about Stevie with you.

''Some years back, my friends took me to dinner to celebrate my birthday.

''On the way in, I saw Stevie sitting at what would end up being a few tables away from me.

''At the end of dinner, when the cake came out and my table started singing Happy Birthday, I looked over and saw Stevie Wonder singing along.

''That moment blew me away not just in the kindness he showed from across the room, but as a lesson on the kind of joy that's possible to share with others when you join in their song.

'' Ever since that day, I have always done what Stevie did for me, and sing along for anyone's birthday at a restaurant.''

And John pledged to ''sing along'' with all those helping the 'Superstition' hitmaker celebrate his milestone birthday.

Finishing the post - which was accompanied by a photo of him and Stevie together - he wrote: ''Stevie, today I sing along with your loved ones on your very special day. [heart emoji] (sic)''