Mountaineer and adventurer Aron Ralston (James Franco) drives to Utah's Canyonlands National Park for a day of hiking. On foot, he befriends hikers Kristi (Kate Mara) and Megan (Amber Tamblyn), and shows them an underground pool. After swimming, Aron parts ways with the hikers, and continues his hike through a slot canyon in Blue John Canyon. While climbing down, he slips and falls, knocking loose a boulder which flattens his right hand and wrist against the wall. Unable to move the boulder, he tries calling for help but realizes that he is alone. He begins recording a video diary to maintain morale, and uses his pocket knife, with his left hand, to chip away parts of the boulder in order to free his trapped arm. He rations his food and water, in order to survive the ordeal.
After many hours of chipping away at the boulder, he realizes he is not getting any closer to freeing himself. He sets up a pulley system using his climbing rope to try and lift the boulder, to futile attempt.
Days after being trapped, Ralston considers using his pocket knife to cut himself free, but finds the dull blade unable to cut bone. With no water, he is forced to drink his own urine. His vlogs then becomes desperate and depressed. He hallucinates about escape, relationships, and past experiences, including a former lover (ClĂŠmence PoĂŠsy), family (Lizzy Caplan, Treat Williams, Kate Burton), and the hikers he met earlier. He starts seeing the boulder that has trapped him as his destiny.
Ralston realizes that by his knowledge of applying torque, he can break the radius and ulna, letting him amputate his arm in order to escape. He fashions a crude tourniquet out of CamelBak tube insulation, uses a carabiner to tighten it, and cuts tissue soon to his success. He wraps the stump of his arm to prevent exsanguination and takes a picture of the boulder. He then rappels down a 65-foot rockface using his other arm, and drinks rainwater from a small pond in the hot midday sun. During his miles-long hike for help, he stops to admire the Great Gallery at the Horseshoe Canyon (Utah). He meets a family on a day hike, who alert the authorities to Ralston's presence, and a Utah Highway Patrol helicopter is dispatched. Ralston is taken to a hospital where he recovers, and is fitted with a prosthesis, and continues his hobbies of climbing canyons and mountains, along with starting a family of his own.

Question: Why can't Aron cut himself free?
Answer: His dull blade cannot cut through bone.
Alternative answer: The knife blade is too dull