Why does freshly cut grass smell like a watermelon?
cis-3-hexenal is probably the main reason.  It is one of the main chemicals in fresh-cut grass smell (lots of references besides wiki if you google it).  This fairly comprehensive paper on watermelon odorants also says that they find a lot of cis-3-hexenal in fresh cut watermelon.  See tables 3.1, 3.2 and 3.3.