https://twitter.com/Ftvi0HzeuiVbs0i/status/1457651284983496704?s=20
Mumei seems like someone who would randomly do stuff like this, she probably also sucks on bottles when she tries to drink from them.
Ask Mumei for some water after a run in PE class.
She gives you her bottle, but first she takes a sip
"Here Anon, I know you drink a lot so I wanted to refresh first"
When you take the bottle you can clearly see her drool around the bottle top
You drink it while Mumei watches you savoring her drool and refreshing with some fresh water
You give the bottle back to Mumei almost empty
"Next time bring your own bottle anon, so you can give me some..."