What is this stuff?

I just did two days at our church's yard sale. 102 F. My daughter gave me
some of the neck coolers the bike patrol uses. I was underwhelmed. The
pellets in the neckband were 1/16 of the volume of the space they were to
fill. But after the first day, they swelled so much, it looked like a plump
sausage. I was surprised.
What is this stuff inside? It seemed to help a little when worn at the base
of the neck, but after swelling, it didn't retain a lot of water. I kept
three in rotation in a Zip-Lok bag with ice, wearing one, and when it no
longer felt cool, rotating it with a cool one.
I must say, the best part was the dripping of the whole thing down my front
and back and the evaporative cooling from that, but the cool gel was good,
too.
Is there a site that explains the chemical/physics aspect of "this stuff"?
Steve