Splitting things up
You already know this one, but you have probably never asked why. A large log burns far more easily once it is split into small pieces.
Lighting a hearth
A large log takes a long time to catch. Split into small pieces, the same wood lights quickly.
What doctors advise
Doctors tell us to chew our food thoroughly before swallowing it, so that it is digested more easily.
What is behind both of these?
The same thing is happening in both cases — the substance is being broken into smaller pieces. Its mass has not changed at all; only its shape has.
But how can a change of shape speed a reaction up? The answer lies in where the reaction actually takes place.