It's a bit embarrassing to reveal this, but it's too good not to share.
Ikea sells a set of plastic cups that 90% of my friends with kids use (the KALAS tumbler). The other day my 3-year-old figured out that the lid from a Pringles can fits perfectly! The fit is so flawless it's basically spill-proof.
Once you pop, it makes a top! Good lidded cups are hard to find, and, with this hack, the price is definitely right.
Also: the best hacks are slightly embarrassing.
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More: Conversations about cups