Given the task definition and input, reply with output. Based on the information present in the given passage, you need to write a correct answer to the given question. A correct answer is one that correctly and completely answers the question. Do not write answers that address the question partially or incorrectly. The correct answer must be contained in the given paragraph.

Paragraph- Sent 1: The properties of matter depend on the types of atoms that matter is made of.
Sent 2: Matter can exist in two forms.
Sent 3: It can be a pure substance.
Sent 4: This means it is not mixed with anything.
Sent 5: It can also be a combination of different substances.
Sent 6: This means it can be a mix of different types of atoms.
Sent 7: You may recall that atoms differ based on the numbers of protons they contain.
Sent 8: Some atoms contain only one proton such as Hydrogen.
Sent 9: Other atoms contain many protons.
Sent 10: There are many types of atoms.
Sent 11: For example, Helium has two protons.
Sent 12: An Oxygen atom has eight.
Sent 13: Water is composed of a two types of atoms.
Sent 14: Water is made of two Hydrogen atoms and one Oxygen atoms.
Sent 15: There are only so many types of atoms.
Sent 16: These atoms can be mixed into an almost limitless amount of substances.
Sent 17: So what do we call a substance that has only a single type of atom?.
Question: In what two forms can matter exist?.
Pure.