Zhuangzi replies
子非我,
安知我不知鱼之乐 'you are not me, so how do you know I don't know the joy of fish?', and ends by saying he knew it standing there on the bridge.
濠梁 is the bridge over the Hao River, traditionally placed near today's Fengyang in Anhui. The exchange is still quoted when someone claims to know how another feels.