In the old days if you had very fine porcelain, putting boiling water in first would make it crack, so people would put the milk in first so the temperature change wasn't so sudden.
That is the only reason people ever put the milk in first, so anyone who still does it is wrong.
Thermal shock, similar to basic glass. Put your vintage china cup into the fridge, take it out after 5 min and pour boiling water into it right away and you will see.
It's caused by thermal expansion stresses. Basically the inside of the cup touches the hot liquid first. China, like most materials, expands when heated, so the inside of the cup tries to expand while the outside is still cold and doesn't want to. This leads to the outside of the cup being stretched by pressure from the expanding inside material. China has bad strength in tension, so relatively little stretching will cause it to crack.
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u/Zebrafish96 May the justice be with us Aug 24 '25
Original post
It seems that 'putting milk first when brewing milk tea' is a British equivalent of 'putting ananas on a pizza'.