Answer:
The substrate must fit perfectly with an enzyme's active site this in known as the lock and key fit.
Step-by-step explanation:
The lock and key model was given by the Emil Fisher. The lock is represent an enzyme which is complementary to its substrate and substrate is called key.
when an enzyme is used to catalyzed the reaction, there are active sites on the enzymes through which substrate bind. The active site have specific geometry which is complementary to the substrate so substrate perfectly fit into it.
It is resemble with the lock and key. As there is a specific key for a specific lock this model is just like this.