Yawns happen a lot when puppies have overdone the excitement and need their sleep. In this case excitement was in the form of wrestling against their siblings for a spot at the milk bar. Heady stuff when you're not even 3 weeks old.
Sleeping is incredibly important to puppies because while they sleep they grow. Overnight they not only appear to have gotten bigger, they actually do. This is why when you add a puppy to your life - although it's very tempting - you shouldn't really drag them to everyone's house for a visit. If they are not sleeping, they are not developing, or rather, they are trying to learn and develop at the same time - rather hard work for a puppy. It's best to have people visit you for the first little while so that puppy has a chance to take time-outs in their safe zone and it's much easier to establish good habits.
Added to which, a puppy who is asleep is not peeing on your floor, chewing a rug, chasing the cat, unlacing a shoe, unravelling a toilet paper roll, finding your dirty laundry or ripping apart that library book you forgot to return. A good puppy is a sleeping puppy.
A sleeping puppy is perfection - all soft downy fur, shiny black pads, wet little nose, folded up ears, wrinkles of thought, tiny curled whiskers and baby frowns of concern. They are all these things and more when they are awake - the more being a dash of extremely naughty! So love the sleeping puppy with all your heart ... and try call it to mind when you find a room full of shredded library book.