in, into

A verb followed by the preposition in expresses condition or location.

• Grégoire was writhing in agony when he broke his knee.
• Juliette was strolling in the park with her boyfriend.
• The Free Press carrier leaves the newspaper in the mailbox every day.

A verb with the preposition into indicates movement or transformation.

• Juliette strolled into the park to meet her boyfriend.
• The Free Press carrier shoves the newspaper into the mailbox every day.
• I had just stepped into the arcade when I met Joe.
• It was once believed that witches could transform people into frogs, bats and other creatures.
• My younger sister turns into a monster as soon as our parents walk out the door.

Although into is normally used in cases of movement or transformation, there are some exceptions common in informal speech, such as “split in two” or “go jump in the lake.”

• When I complained, Jason split the last piece of cake in two.
• Annoyed by her brother’s teasing, Miranda told him to go jump in the lake.