context: One fine spring afternoon, Joe (Daniel Craig) and his girlfriend Claire (Samantha Morton) get away for a romantic picnic in the countryside near Oxford. Joe pauses before he surprises Claire by producing a bottle of Dom Perignon. The moment appears portentous, but before Joe can pop the cork and say whatever is on his mind, a huge red hot-air balloon drifts onto the field.A young boy cowers in the basket of the balloon which is being dragged along the ground quickly while an older man, his grandfather, trails behind, pulling on one of the ropes and urging the frightened boy to jump out. Joe races to assist, and he is joined by several other men coming from all directions. It looks like they've got the situation under control, but before the boy can climb out of the basket, the wind picks up, setting the balloon suddenly aloft; the men hang on reflexively. As the balloon rises higher and higher, four men let go one by one, tumbling safely to the ground. They recover from their panic only to realise that one man never let go and is now dangling from the end of a rope hundreds of feet in the air, the balloon still rising and floating away from the field. The group watch in helpless horror as the man finally loses his grip and falls to his death in the distance.While the onlookers all gape in shock, Joe, rational and practical to a fault, decides to walk over to where the man fell and see if anything can be done. One of the men from the group (Rhys Ifans) follows behind him. A grisly sight awaits Joe and the other man, and it is immediately obvious that the man who fell is quite beyond assistance. The man who tagged along suggests that Joe pray with him. Joe resists, grumbling that prayers won't help, but the other man pleads with him until he finally kneels down. Eventually the authorities arrive and everyone leaves the tragic scene.Shortly afterwards, Joe and Claire are dining at the home of their married friends Robin (Bill Nighy) and Rachel (Susan Lynch), retelling the story of what happened on that fateful afternoon. Joe expresses guilt at having let go of the balloon: if none of the men had let go, he reasons, their combined weight would have kept the balloon from soaring upwards the way it did. He also struggles with the senselessness of the accident-- the boy ended up landing the balloon safely on his own, so the dead man, a doctor named John Logan, sacrificed his life for nothing. Claire's insistence that "it was the wind" only irritates Joe, who can't accept the random and bizarre nature of his traumatic experience.While Joe is at home one afternoon, ostensibly working on his writing but in actuality obsessing about the balloon accident, the phone rings. It's the man who accompanied him to find the doctor's body. He tells Joe he'd appreciate talking over what happened that day. Joe is eager to commiserate with someone who can understand what he's feeling and agrees to meet the man. As it turns out, the man is already standing right across the street from Joe's apartment and waves to him as he looks out of his window.Joe crosses the street to a playground where he reintroduces himself to the man, who gives his name as Jed. Joe wants to discuss the details of the accident but Jed hems and haws, hinting obliquely that he knows what Joe wants to "come out and say." Joe has no idea what Jed's point is and eventually excuses himself, confused and a bit annoyed.Joe teaches a class at college where he tells students that love is nothing more than biological impulses dressed up in sentimentality.Eventually Jed catches up with Joe again at a bookstore, still acting as though he and Joe are on familiar terms and moreover that he's waiting for Joe to tell him something that they both know Joe wants to tell him. Joe manages to get away from Jed, but sees him again days later, lurking at a nearby table at a restaurant where Joe's lunching with his agent (Andrew Lincoln).While Jed seems obsessed with Joe, Joe remains obsessed with the balloon accident. Hoping to gain some insight that would put everything in a better perspective, he visits Logan's widow (Helen McCrory), only to learn that since the death of her husband, she's been agonizing over his last moments alive, wondering whether he spent them out in the country on a tryst with another woman. She found the remains of a picnic in his car as well as a woman's perfumed silk scarf. She grills Joe to remember whether there was one door open when her husband exited his car to rush to the accident, or two, indicating that he had a passenger. Joe can't remember and all he cares about is who let go of the balloon first, a question that only serves to upset the widow Logan further. Neither she nor Joe comes away from their meeting with any greater sense of peace.Jed's increasingly aggressive harassment of Joe combines with Joe's post-traumatic stress until he unravels. He gives a class lecture that sounds more like an angst-ridden rant and he alienates Claire with his self-absorbed and morose thoughts. Finally Claire leaves Joe. Jed tells him that it's for the best because now they're free to be together.Angry and desperate, Joe manages to obtain Jed's address from Mrs. Logan. He creeps into Jed's seedy apartment and finds ample evidence of Jed's obsession with him. When Jed appears, Joe lunges at him, coming close to physical assault before he warns him to stay away and leaves. While Joe cools his rage with several drinks in a bar, Jed beats his own head against the wall.Joe stumbles drunkenly through the rain to Robin and Rachel's happy family home and since Claire's thrown him out, they have no choice but to let him sober up at their place. The next morning, Robin tells Joe that Claire phoned to say that she was with someone named "Jed." Upon hearing that name, Joe leaps to his feet, dashes out of the house, hops in his car and speeds home. A bruised and beaten Jed lounges in Joe's bathrobe as Claire glowers at Joe. Joe can't believe that Claire has sided with Jed.Joe's ready to leave in disgusted defeat when Jed suddenly takes a kitchen knife and stabs Claire in the belly. Claire collapses, a pool of blood slowly expanding from her wound. Joe thinks quickly. He tells Jed that they can be together now and lures him into an embrace. As soon as Jed's guard is down, he grabs the knife and stabs Jed, then runs to Claire's aid.In an epilogue, it's now autumn. Joe has asked Mrs. Logan and her daughter to accompany him to the field where her husband died. He introduces her to an older man and a young blonde woman. The couple explain that John Logan had spotted them on the road after their car broke down and offered them a ride. When the accident happened, they ran off and left their food and the girl's scarf behind. Mrs. Logan sobs with relief and guilt over having mistrusted her dead husband. Claire, now walking with a cane, also shows up to see Joe. They seem to have mended their relationship somewhat, although they now see each other differently. Joe tries to recapture the moment when he originally had planned to propose, opening the same bottle of champagne, but Claire stops him. "Don't say anything," she says, as she sips her champagne. "Don't say anything."
question: What does Joe teach his students about love?
answer: It is nothing more than biological impulses dressed up in sentimentality.