Happy to be told I'm doing it wrong, but I think I might have a faulty one.
I measure out the hot water in DD's bottles then pour all but about 1oz into the rapid cool and flip it a few times. Even with boiling water in there, it never goes red, only ever amber. I'll flip it, shake it for a minute or two and then pour it back into the bottle (with formula mixed into the hot shot). It is ALWAYS too hot when I test on my wrist, and I end up having to put the whole bottle back in for a second time.
Am I doing something wrong? As it never goes red I'm inclined to think I've got a faulty one.