Final answer:
The green substance you described behaves like a liquid, changing shape to fit its container while maintaining a constant volume, which makes its shape irregular when not in a container.
Step-by-step explanation:
If you encounter a green substance that changes shape based on the container in which you put it, this behavior is characteristic of a liquid. According to the states of matter, solids have a fixed shape, while liquids take the shape of their container and have a definite volume. Gases expand to fill their container in both shape and volume. Since our substance takes the shape of both a round and a rectangular container while presumably maintaining the same volume, we can deduce that it is a liquid, which corresponds with one of the three common states or phases of matter: solid, liquid, and gas.
Therefore, the shape of the substance is not fixed and depends on the container it occupies, making irregular the correct answer to the question 'What is the shape of the substance?'