Why does Pepsie go flat in minutes?

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I remember 20 years ago a can left for a week open would still have fizz.....now a dats the fizz is gone before i have finished the can of pepsie. Does ths save pepsie money? I wonder.....anyone else mad that this happens? It tastes like crap after the fizz is gone. I bought the clip on bottle with screw on cap that goes over the can and allows it to be clossed but it did not work as it claimed and the fizz was gone all too soon! I can't be the only one to taste the difference...let me know!!!