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It's canonically stated that Lockwood doesn't like sugar in his tea. (I believe it was in the Creeping Shadow)
So I now have a firm belief that Lucy puts sugar in Lockwood’s tea whenever she's pissed at him. He might already know she's upset, so sometimes the sweet taste is not a surprise to him. But other times, when he's unaware of what she's feeling, he sips his tea and gets a surprise -oh f*ck I did something what did I do???- moment.
The amount of sugar also depends on the severity of what he did. Sometimes he doesn't even taste it, there's so little. Other times his tea is the consistency of mud.
One time in particular Lucy offered him his cup just as she always did. He took it with thanks and went to take a drink when he looked down.
There was no tea.
She handed him a cup of sugar.
Lockwood had never been more afraid in his life.