how can usui's KISS on ayuzawa's back "while she was wearing a bikini" leave a mark ???

i watched the Anime and read the Manga for about 8 times in total but still cant understand,,,,i know i am stupid but just till me: how can usui's KISS on ayuzawa's back "while she was wearing a bikini" leave a mark ???
Anime : episode 16 -or 17-
Manga : chapter 18 -or 19-