Perplexed.

This morning I poured myself a glass of tap water. It was perfectly clear. As I sat there (doing the Sunday crossword - a failure to finish sadly) I noticed the bubbles appearing. 

What mechanism generates them? What gas do they consist of? Have I nothing better to do?

This afternoon I shall ask Mr Googles. 

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I did ask Mr Googles "why do bubbles appear in glass of tap water?" I am now older and wiser, everyday a school day.