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It’s to keep them stable/bottom heavy. Lots if panicked mums returning their cups now, though.
Edit @Greenbank has corrected my misunderstanding:
Stanley say a lead pellet is used internally to seal the vacuum installation. I'm guessing because it has a very low melting point.
Leshaches
dubtap
Greenbank
Turns out Stanley Cups only have a tiny lump of lead in them lol