Why do Japanese people decorate carp streamers?

You might see some carp-shaped windsocks decorated in Japan at this season. Do you know why?

They are called 鯉のぼり “Koinobori” and we fly them in honor of children for a good future and in the hope that they will grow up healthy and strong.

Usually those Koinobori are decorated at the houses who have sons, but today I saw a lot of them flying on the river in Okazaki. It was awesome!