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It takes a while to get the knack of it but i rip the top off and hold it under a small waterfall or face the hole into the flow and it fills op pretty quickly.

Forgot to mention They also make a good pillow if you blow one up with air

I use the bladder out of cask wine boxes. They fit over 3 litres in them but i only fill them as much as required to cut down on weight.
They squash down to bugger all when empty and they are...