She's only six-weeks-old, but this twin knows how to stop her brother's tears.

A tender moment has been captured by a US mum using the baby monitor she had set up to observe her sleeping twins.

User “Twin Mom” uploaded the video on YouTube after she observed her twins sharing an unusually sweet solution to an age-old problem.

The two six-week-olds are locked in a gentle embrace on the cusp of sleep.

When the boy on the right begins to cry, the camera captures the kind offering of a thumb by his twin sister.

The boy accepts the thumb and is able to soothe himself.

When the thumb begins to fall out of place, he uses his left hand to make a gentle fist to place it back.

The video has only been online for a week but it has already clocked up over 27,000 views.

The mother who posted it also left a sweet explanatory note behind the behaviour.

"My six week old fussed when I went in the other room so his twin sister took care of the situation," she wrote.

Other users have commented on the video with a few of their own theories.

"Maybe they are used to do this from the womb!" user moreychiquita wrote.

"She probably wanted him to be quiet so she could sleep, so cute :)," user Lucie Locket wrote.

You can watch the video below...

The jury is out on the true motivations behind the technique but considering how sleepy the child on the left seems, it may have been more opportunistic than intentional.

Watch as the team at MM share some of the strangest (and sweetest) things our children have done.