The Black Stallion

Alec Ramsey is aboard the steamer Drake off the coast of North Africa, where he sees a wild black stallion being forced into a makeshift stable and heavily restrained by ropes leading to his halter. Captivated by the horse, Alec later sneaks to the horse to feed him some sugar cubes, but he is caught by the horse's supposed owner, who tells him in Arabic to stay away from Shetan and shoves the boy away.
Later in his bunk, Alec's father shows Alec his winnings from a card game and gives him a pocket knife and a small statue of Bucephalus, and tells the story of how Alexander the Great became Bucephalus' master. Later that night, Alec is thrown out of his bunk; the ship has started to capsize. In the chaos, Alec grabs his knife and his Bucephalus statue, and makes his way to the black stallion and manages to free him. The stallion then jumps into the sea. Alec himself is swept overboard by a gigantic wave. Once in the water, he swims toward the stallion and managed to grab hold of the ropes of the stallion's restraints.
Alec wakes on the shore of a deserted island and starts to explore. He finds the stallion caught in his restraints with the ropes stuck between the rocks. With his knife, Alec manages to free the stallion once again and the stallion runs away. For a time, the two keep their distance. Alec discovers means to survive by catching fish and seaweed. As Alec suddenly faces a cobra eye to eye, the Black comes to the rescue and kills the snake, only to run off again.
By now, Alec decides to try to get closer to the horse and offer him some seaweed. The hungry stallion finds himself unable to resist, but visibly struggles with his distrust for humans. Eventually, the hunger wins and he takes Alec's offer; their bond has been sealed and the two are now inseparable. Alec even manages to ride the unbroken horse, after many times falling off the horse. One day, a fishing ship arrives, rescuing both Alec and the stallion.
Back home, Alec is given a hero's welcome. The Black has a temporary home in Alec's back yard, but a garbage man not knowing that there is a wild horse in the back yard is chased by the Black, who races off down the street after being spooked by a passing car. Alec chases after him through every part of town, but loses track of him. The next day, Alec meets Snoe (and Napoleon) who tell him where the Black is. Alec finds the stallion in the barn of Henry Dailey, a retired racehorse jockey, who apparently spent all night catching the Black. Alec arranges for the Black to stay at the barn.
When Alec wonders how fast the Black is, Alec and Henry decide to train the Black for the racetrack, while Henry teaches Alec how to be a jockey. The Black surprises Henry with his speed. Henry immediately starts plotting to get the Black into a match race between two of the country's current champions, to be held at Santa Anita, California. To do that, he sets up a secret demonstration at night where the press can witness his speed, keeping the identity of Alec and the Black secret. The news about the mystery horse is soon widespread and the Black is entered into the race.
The race is the most anticipated horseracing event of the year. Before the two champions and the Black enter the starting gate, the Black gets into a fight with one of his opponents, wounding his leg. Alec does not see the wound until he is in the gate. As he dismounts, the bell rings and the horses take off. Alec desperately tries to stay on his horse and trying to stop him. He falls behind, but the Black won't stop. When Alec regains his balance, the Black is well on his way to catch up with his opponents. Alec now encourages the Black to run as fast as he can, remembering the wild rides on the island, as they catch up. The Black eventually wins by two lengths.

Question: What does Alec use to free the stallion?
Answer: knife