I opened one for the first time yesterday.
It took the strength of me and my brother, then we almost gave up until i gave it another try, then it practically blew up in my face and i lost a fraction of the drink.
You let the bottle rest for a bit, then you take off the cap and take the weird plastic thingie and press down on the marble with it, and then you let it stay there for a few seconds while pressing down and then you take the plastic thing out
When i first did the exact same thing, it still practically blew up.
I opened another one around an hour ago and it actually went well and was pretty easy.