It's impolite to pick at you nails in public.
Kelly is on the bus riding it back home when she notices how dirty her nails are.
Kelly wants to pick the dirt out of her nails.
Kelly waits until she gets back home to clean her nails.
Kelly starts picking the dirt out from under her nails on the bus.
The other passengers don't notice how filthy Kelly's nails have become.
The passengers near her are grossed out by what she is doing.