Sentence:  When they reached their neighbor 's house , Pamela said a few polite words to Grace and <WSD>kissed</WSD> Melissa lightly on the forehead , the impulse prompted by a stray thought - of the type to which she was frequently subject these days - that they might never see one another again . 
Meaning of the word marked with <WSD> tags:  touch with the lips or press the lips (against someone's mouth or other body part) as an expression of love, greeting, etc.