Here's why:

* Active Site: This is a specific three-dimensional region on the enzyme's surface that is designed to bind to the substrate. It has a unique shape and chemical properties that are complementary to the substrate.

* Substrate: This is the molecule that the enzyme acts upon.

Think of it like a lock and key:

* The enzyme is the lock.

* The active site is the keyhole.

* The substrate is the key.

The substrate fits perfectly into the active site, allowing the enzyme to catalyze a specific chemical reaction.