Here's why:

* Hypotonic: A hypotonic solution has a lower concentration of solutes (like salts and sugars) compared to the inside of the cell.

* Hypertonic: A hypertonic solution has a higher concentration of solutes compared to the inside of the cell.

* Isotonic: An isotonic solution has the same concentration of solutes as the inside of the cell.

Since pure water has no solutes, it's automatically less concentrated than the inside of a cell, which contains various solutes. This means the water outside the cell has a lower solute concentration, making it hypotonic.