erm....where are you getting the cockles from?
I use the gamma ones from TMC, drop them in some hot water, then prise them open with a knife, once completely defrosted, i crack them up a little and drop them in the tank.
lol, right ok. So you crack them enough for the puffer to get in without having to "work" too hard but not enough for them just to eat the meaty bit straight out the shell. If you get what i mean