The tough base of this mushroom is very hard.

Studying the history of the Middle Ages is very interesting.

We need mutual understanding.

He believes in 'an apple a day keeps the doctor away', so he eats fruit every day.

Research on xenotransplantation has the potential to open new frontiers in medicine.

This river has a lot of small stones.

This rock is a type called nepheline.

He is an expert in ethology, and his research is highly regarded worldwide.

He starts talking about himself at every opportunity, causing trouble for the people around him.

He likes to take a walk in the park at dusk.