What do you get when you cross an absent minded elephant with a small flea?

A Forget Me Gnat

This is a joke that is based upon the use of a pun.  The use of puns is intended to be humorous because of how words can sometimes have different meanings or how words can be similar sounding.  The answer to this joke is “a forget me gnat.”   Elephants are known to have really good memories; thus, an elephant would tend to not forget something or someone.  A “forget me not” is type of flower that one would typically give someone in the hopes of always being remembered.  A small flea type of bug is a gnat.  The pun here is how “not” and “gnat” sound similar and how “gnat” is used instead of “not.”

several tissues working together form an organ, and several organs together make up an organisms.

the group of cell that carries out special jobs are tissue.