A built-in straw needs a teardown, not a rinse

You need to take the straw or valve apart, scrub the pieces, and let them dry fully — otherwise you’re breeding mold.

A built-in straw or valve is basically a tiny dark, damp cave. Juice, milk, and crumbs get in there, and a quick rinse under the tap doesn’t touch the gunk. The valve especially: it’s designed to seal, so it also seals in moisture. That’s how you end up with a lunch box that smells faintly of sour yogurt even after you clean it.

The good news is most manufacturers make these parts removable for this exact reason. Pop the straw out of the lid, pull the silicone valve free. If you can’t take it apart, you’ll have to be even more aggressive: run hot soapy water through the straw, then follow with a narrow bottle brush (or a pipe cleaner if it’s thin). For the valve, soak it in a vinegar-water solution for 15–20 minutes to break down any film.

The last step is the one people skip: dry it completely. Leave every piece on a drying rack or paper towel with the straw standing upright, and don’t reassemble until it’s bone dry. If the manufacturer says it’s dishwasher-safe, you can toss the pieces on the top rack, but even then, I’d hand-dry and let the valve sit out overnight. That extra five minutes is all that stands between you and a fun science experiment.

Trust me — you don’t want to find out what’s growing in there.