Who pissed in your cornflakes?

Meaning

Functionally identical to the Cheerios variant, this is used to point out someone's bad attitude at the breakfast table or generally throughout the day. It is meant to be funny enough to snap the person out of their funk.

Cheeky, casual

Used in a sentence

• You are acting like the sky is falling; who pissed in your cornflakes?
• I am just trying to do my job, so tell me, who pissed in your cornflakes?
• My brother came down to breakfast and started yelling about the milk, so I asked, who pissed in your cornflakes?