Rats that eat gold

Zhang SAN and Li Si are neighbors.
One day, Zhang SAN said to Li Si, "I am going out to do business. You should take good care of my family's gold." Li Si readily agreed.
After a few days, Zhang SAN came back. Said to Li Si," Where's my gold?" Li Si lowered his head and said, "Your gold has been eaten by mice." Zhang SAN turned and walked away without saying anything.
In the night, Zhang SAN stole Li Si's horse.
The next morning Li Si said to Zhang SAN, "My horse has been stolen." Zhang SAN smiled and said, "I saw an owl fly away with a horse on its back last night."
Li Si said, "That's impossible. How can an owl carry a horse?" Zhang SAN smiled and said, "A mouse can eat gold, but an owl can't carry a horse?"
Hearing this, Li Si was very ashamed.
The next day, Li Si returned the gold to Zhang SAN, and Zhang SAN also returned the horse to Li Si.