cuz... it was her birthday and pratchett first sang her a 'happy birthday' and then said:
-uh.. since it's your birthday.. i will let you kiss me *huge grin*
the girl was all, like: OMG OMG OMG that's the best birthday present EVER!!! TERRY PRATCHETT sings 'ahppy birthday' for me and i get to KISS HIM!!! OOOOOOOOOOOOOMFG
and stuff like that D: