Now I have a vision of a creationist workshop and the hapless apprentice saying "I've finished the merpeople and let them go, what can I do now?"
The great creator "come and help pop the the duck beaks and beaver tails in the stoat bodies please"
Apprentice "like this?"
Creator "No, you have to twist them on until they click"
Apprentice "ahh, erm were the merpeople tails the same because I just did them finger tight"
Creator "You should have turned until they clicked, their tails are on sideways now"
Apprentice "well looked at the whales and dolphins that you did and copied them".
Creator "no, no they have tails that fit completely differently, they go up and down with a horizontal fin. The mers had fish tails, should have been a vertical fin and side to side to swim. Oh well too late now, we just have to see if they work OK."