Milk is bacterial food. Leave lactose and fat inside a closed plastic cup at room temperature and the bacteria already on it double every few hours ¹. Within a day or two they stop floating and glue themselves to the plastic as a slimy biofilm that's 10 to 1,000 times harder to kill than loose bacteria in liquid ².

The cup's own design hides them. The valve under the spout sits flat against the lid, so tap water goes around it. The straw bore is the width of a pencil and never gets rinsed inside. The gasket seals in a groove nothing reaches until you pop it out. The crevices that make the cup spill-proof are what make it an incubator.

Rinsing hard doesn't help. Swabbed bottles in Lima grew E. coli inside 44% and coliform bacteria in every single one; whether caregivers said they boiled or used detergent predicted nothing, and only guided brushing did ³. In UK homes, over half the bottles parents believed they'd sterilised still carried bacteria ⁴.

The one non-negotiable is that a cup left assembled cannot be cleaned. Take it fully apart, every time.

Three groups need daily sanitising on top: infants under two months, babies born early, and any child with a weakened immune system. For them, boil the parts 5 minutes, steam-sterilise, or soak in 2 teaspoons unscented bleach per gallon for 2 minutes and air-dry without rinsing. The methods are roughly equal; pick the one you'll actually do ⁶. A healthy toddler past two months needs only the wash-and-dry.

The headline benefit is one you can't see: the bacterial GI episode that never happens. Most toddler stomach bugs are viral and come from peers, and cleaning doesn't touch those. It closes off the rarer, worse fraction, the Cronobacter and E. coli that send a small child to urgent care for fluids ⁷. The benefit you will notice, within a week, is smaller: you stop opening forgotten cups with dread, and the "is that mould?" check under the gasket stops finding anything.