YOU SAID:
Tickle his kneecap to make him wet
INTO JAPANESE
膝をくすぐって彼を濡らす
BACK INTO ENGLISH
Tickle your knees to wet him
INTO JAPANESE
彼を濡らすためにあなたの膝をくすぐる
BACK INTO ENGLISH
Tickle your knees to wet him
That didn't even make that much sense in English.