Punning, using wit or joke to name the object in association with its character or history. For example, the chashaku on the far left was named Kamakiri ('Praying mantis') and the second from the left is  Imomushi ('Green caterpillar'), based on a combination of the look of the chashaku and the different personalities of the two people who carved them.

Image thanks to Jun Tanimura: @leoniki on Instagram.