Specialised cells grouped together with the same function are described as tissues. Muscles cells are given as an example. An organ is described as a group of tissues with the same function. A heart is given as an example.
This clip should be used to explain the difference between cells, tissues and organs. It opens with the question 'What is the difference between a tissue and an organ?'. It therefore lends itself very nicely to reviewing this question at the end, following a discussion of how an adjacent group of identical cells working together is a tissue and how two or more different adjacent tissues working together comprise an organ. It could be extended to explain how organs comprise systems.