"Cracks a Yolk" is the second episode of season four and the forty-first episode overall of the animated children's series The Magic School Bus. It originally aired on September 20th, 1997.
Dorothy Ann is taking care of one of a chicken egg and becomes impatient when it has yet to hatch after she accidentally lost Mr. Rhule's Rhode Island Red rooster, Giblets. A flashback shows her promising him she will take good care of Giblets and he flies away. When the scene fast-forwards to the present, Dorothy Ann gets impatient and the class is eventually reported to Mr. Rhule's office. So she decides that they should tell him the truth and that they are getting him a new one by hatching one from an egg. He, however, takes this as a joke and decides to play along. They tell him that they got a Rhode Island Red egg to a farm replete with Rhode Island Red chickens and Dorothy Ann tells him that the farmer let them get one as long as the chickens didn't escape.
Another flashback shows the class at the farm and comes across a pen with males called roosters and females called hens. Ms. Frizzle guides the class into it, where they get pecked and eventually run out. Dorothy Ann comes across a hen pen and decides that they should get the egg from there. She walks inside and grabs one from inside the henhouse. But she trips, it flies out of her hand, and cracks against a pan Liz holds out to crack the yolk onto. Dorothy Ann runs out of the pen and tells Ms. Frizzle that it was going to be Mr. Rhule's new chick. To find one with a chick inside it instead, Ms. Frizzle has Liz use the bus to shrink them inside and cause them to fly inside a hen.
Back in Mr. Rhule, where the story is being told, Mr. Rhule finds it hard to believe the class was actually inside a chicken in search for an egg. Dorothy Ann continues her story, stating that they started in the ovary.
Back in the flashback, the class find themselves inside the ovary, where Ms. Frizzle explains to them that is the place in which the eggs start ticking. They then notice big, white spots on each of the orange balls in there and Ms. Frizzle explains that those are the most important part of the eggs. Just then, a yolk falls out of one of the balls, and they slides down a tube Ms. Frizzle says is called the oviduct, with the yolk trailing behind. They land right on top of it, which continues to slide down the oviduct, and they notice it get covered with white layers and get tangled with some stringy substance.
Back in Mr. Rhule's office, the class tells him that a layer formed around the yolk and white and swelled up into the shape of an actual egg. he then learns that it is exactly what happens when an egg is completely formed.
As Dorothy Ann continues telling the story, the class notices solid substances forming around them until everything gets covered in a hard shell. Inside the egg, they search for a chick and comes across an ovum, which is half of what starts one, meaning this egg won't hatch inside Mr. Rhule's chick either. So Dorothy Ann decides that they should search for another one immediately.
Back in Mr. Rhule's office, he finds it very hard to believe that the hen laid an egg with the class inside it. Dorothy Ann tells him that they did and they would still be inside it had it not been for Liz.
In the story, Liz steals the egg out of the hen's nest and uses a flashlight to see the class inside through it. She runs out of the pen, cracks the egg against her head, they slide off, and returns back to their normal size. Phoebe informs them that the egg they were in came from the hens instead of the roosters and Dorothy Ann then learns the other half of the chicken is the rooster and that it takes a mother and father to create a child. They walks into the rooster and hen pen and they learn the hen makes the egg while the rooster adds his part of it before it grows into a chick. So Dorothy Ann acquires one from a nest and they takes the bus back to school. Inside the bus, they wonders if only the hen's half is inside the egg and no rooster came along on time. So D.A. decides to have Ms. Frizzle shrink her to size to get inside it. But when Arnold tells her that he would rather go inside it than face Mr. Rhule, Ms. Frizzle shrinks him instead and he ends up inside the egg. Back in the classroom, he is searching for a chick and comes across an ovum.
The next scene shows the rest of the class inside Mr. Rhule's office as Dorothy Ann finally concludes the whole story and he, having enough of this, tells them he wants his old or new chicken immediately, also adding that it takes three weeks for one to grow, much to Dorothy Ann's horror as it will not grow in time.
Back in the classroom, Dorothy Ann explains the situation to Ms. Frizzle and Arnold, still inside the egg, informs that the chick just got started. To make it grow faster, Ms. Frizzle transforms The Magic School Bus into a hen and speeds time to cause the chick to grow quicker. Arnold then notices that a new chick is growing a head and eyes and Ms. Frizzle tells him that it is called an embryo. He then realizes that its heart is beating and blood is flowing through the yolk. The class arrives back in the classroom and Phoebe is told to tell Mr. Rhule everything Arnold is saying is happening inside the egg using a tape recorder. She explains it all to Mr. Rhule as the embryo grows arms, legs, and even feathers. As its gets bigger, the egg finally hatches and the chick, as well as Arnold, breaks out. Liz uses the port-a-shrinker to return Arnold back into his normal size and Dorothy Ann, out of gratitude, kissed Arnold, causing him to blush. Mr. Rhule arrives in the classroom just in time to see the chick, and Giblets also arrives back, much to their surprise. Mr. Rhule believes it was all a joke and guesses that they were just only hatching eggs the whole time to show him what they learned. Ms. Frizzle lets Arnold and Dorothy Ann keep the chick (which Arnold names Arnold Junior) and she concludes, "You can count your chickens before they hatch...as long as you know what kind of egg they've been in", causing everyone to laugh.
- The only kiss in the series happens when Dorothy Ann kisses Arnold on the cheek to thank him for getting the chick to hatch and for saving her from getting into trouble with Mister Rhule. Arnold Perlstein proceeds to blush due to this.
- Wanda's Yolkahontas joke is a pun on Pocahontas.
- During the scene inside the chicken's reproductive system, Ralphie's cap is on his helmet.
- This episode compared to other episodes depicts the field trip in a flashback.