This one surprises people every time.
Wool fiber is hollow. Actually hollow. There's a canal running through each individual strand that traps air, and trapped air is what insulates. It's the same principle as a down jacket or a double-pane window. Air doesn't conduct temperature well, so it keeps warmth in and cold out.
But here's where it gets interesting: that same hollow structure works in reverse. In warm weather, it wicks moisture away from your skin and releases it as vapour. The fiber breathes. It moves humidity out before your body registers it as sweat.
This is why wool has been the fabric of choice for centuries of outdoor workers, sailors, shepherds, and alpine athletes. Not because it was the only option. Because it does both things at once, and very little else can say the same.
A Bumby cover sitting against a baby's skin in July is managing temperature, not trapping it. In January, same fiber, same cover, keeping that tiny body exactly where it needs to be.
It's not magic. It's hollow. But it does feel a little like magic.