eli5: Why does shaking a bottle of soapy water increase the pressure inside?

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If I partially fill a bottle with water (even cold water), add some soap, close it up and shake it; when I open I open it back up, it tends to spurt / hiss / generally act like it was under pressure all of a sudden, even though the total volume of liquid inside is still the same.

Why?

Please and thank you xxx

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