Rouleaux are small groups of red blood cells stuck together.
Rouleaux are a normal thing in blood, but if there are too many, it can mean something is wrong.
Examples:
The doctor looked at the blood and saw rouleaux.
Rouleaux are in the blood and help it do its job.
The blood had rouleaux in it, so the doctor knew something was wrong.
The rouleaux in the blood were having a party!
In the monster's blood, there were so many rouleaux that it looked like a traffic jam.
The rouleaux in the blood were all stuck together like a big family.