Advantage - what goes on inside that tube?

[LadyWellian]

I've just been trying to squeeze it on to my cat's neck and I can't work out if nothing's coming out (and, say, I need to pierce it) or if it's just invisible?

Have you turned the lid upside and put it in to tube twisted and broken the seal?

Lizcat I hadn't, but having now found the instructions am planning to do so next time the cat is in the vicinity.

Thanks!