In my previous post, I shared the wool felt rabbit that I made. The needle that I used for hardening and joining the wool felt is unlike normal needle that we use for sewing. The needle have pricks on its side. I think that is reason why the wool felt will harden when needle are used.
This is the needle provided inside the kit.
The side of the needle have pricks on it. The needle is prone to break if you didn't poke it into the wool felt vertically.