Will You Be Shopping At Boots This...
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No best answer has yet been selected by tartwitahart. Once a best answer has been selected, it will be shown here.
For more on marking an answer as the "Best Answer", please visit our FAQ.This is basic science - the sort you learn in school and you find 'soo-oo-oo boring'. (qv your question in 'science')
"Fizzy drinks" are water with a gas called carbon dioxide (CO2) dissolved in it. The gas is kept in solution by the bottle top or closed can. Once opened the gas starts to exsolve or come out of solution ('fizz') - eventually going 'flat'.
When you drink your sugar-laden concoction, the carbon dioxide comes out of solution in your stomach, increasing the gas pressure within it. The increased gas pressure tries to 'force itself out', making you burp.