To become hard and thick, like a shell or a bone.
Sclerotise can make people think of protection, strength, and durability.
Examples:
Over time, the tree's trunk will sclerotise and become very strong.
The caterpillar will sclerotise and turn into a chrysalis.
The turtle's shell is sclerotised to protect it from harm.
I wish I could sclerotise my homework so it wouldn't get crumpled in my backpack.
The superhero's suit is sclerotised to make it bulletproof.
The chocolate will sclerotise and become hard if you put it in the fridge.