2 added some much needed space in between sentences
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The problem is occurring because some of the CO2 is escaping into the air cavity at the top of the bottle which grows every time you pour more out. The gas pressure is greater in the liquid and it will equalise with the space in the bottle. 

You need to somehow reduce the space in the bottle. You can do this with a plastic bottle by squeezing it, but this is actually counter-productive as escaping gas will squeeze it back out again. 

So, you can transfer it to a smaller containertransfer it to a smaller container or you could put something solid in the bottle put something solid in the bottle (maybe baking beans - small food-grade ceramic things used for weighing down pastry before adding the filling). The problem with both of these approaches is that agitating the drink causes it to lose CO2 quicker. 

One final thing you could try is adding a pinch of sodium bicarbonate (baking soda) to the bottle before quickly closing it. This will react with the acid in the drink (won't work with mineral water) and generate CO2 in the sealed bottle and may or may not change the flavour perceptibly.

The problem is occurring because some of the CO2 is escaping into the air cavity at the top of the bottle which grows every time you pour more out. The gas pressure is greater in the liquid and it will equalise with the space in the bottle. You need to somehow reduce the space in the bottle. You can do this with a plastic bottle by squeezing it, but this is actually counter-productive as escaping gas will squeeze it back out again. So, you can transfer it to a smaller container or you could put something solid in the bottle (maybe baking beans - small food-grade ceramic things used for weighing down pastry before adding the filling). The problem with both of these approaches is that agitating the drink causes it to lose CO2 quicker. One final thing you could try is adding a pinch of sodium bicarbonate (baking soda) to the bottle before quickly closing it. This will react with the acid in the drink (won't work with mineral water) and generate CO2 in the sealed bottle and may or may not change the flavour perceptibly.

The problem is occurring because some of the CO2 is escaping into the air cavity at the top of the bottle which grows every time you pour more out. The gas pressure is greater in the liquid and it will equalise with the space in the bottle. 

You need to somehow reduce the space in the bottle. You can do this with a plastic bottle by squeezing it, but this is actually counter-productive as escaping gas will squeeze it back out again. 

So, you can transfer it to a smaller container or you could put something solid in the bottle (maybe baking beans - small food-grade ceramic things used for weighing down pastry before adding the filling). The problem with both of these approaches is that agitating the drink causes it to lose CO2 quicker. 

One final thing you could try is adding a pinch of sodium bicarbonate (baking soda) to the bottle before quickly closing it. This will react with the acid in the drink (won't work with mineral water) and generate CO2 in the sealed bottle and may or may not change the flavour perceptibly.

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The problem is occurring because some of the CO2 is escaping into the air cavity at the top of the bottle which grows every time you pour more out. The gas pressure is greater in the liquid and it will equalise with the space in the bottle. You need to somehow reduce the space in the bottle. You can do this with a plastic bottle by squeezing it, but this is actually counter-productive as escaping gas will squeeze it back out again. So, you can transfer it to a smaller container or you could put something solid in the bottle (maybe baking beans - small food-grade ceramic things used for weighing down pastry before adding the filling). The problem with both of these approaches is that agitating the drink causes it to lose CO2 quicker. One final thing you could try is adding a pinch of sodium bicarbonate (baking soda) to the bottle before quickly closing it. This will react with the acid in the drink (won't work with mineral water) and generate CO2 in the sealed bottle and may or may not change the flavour perceptibly.