Private journal

Private journals, or diaries, are a feature in The Sims 2 and The Sims 4: Parenthood.

The Sims 2
In The Sims 2, any child or older can write in their personal diaries, available from any bookcase with the interaction "Write in Diary". This interaction raises fun and enthusiasm in the Film and Literature hobby.

Sims may stop writing and hide their diaries when another Sim enters the room they are in, and will not resume until the room is empty again.

By default, Sims write with scalpels instead of pens, which can be fixed using this mod.

The Sims 4
In The Sims 4: Parenthood, Sims can write daily diaries that are usually kept from other Sim's prying eyes. Private journals can be bought from the Activities and Skills menu.

Sims who write private journals can get a "Another Day: Another Entry" happy moodlet for four Sim-hours after writing. Children and teens can get a great boost to their emotional control character value after writing in their journals.

Private journals can also be hid under beds and other Sims can try to snoop them. If successful, they can get a reduction to their empathy value and a confident moodlet. If the journals owner notices another Sim snooping their journal, they will get an angry moodlet, while the Sim snooping becomes embarrassed.

Sims from child to elder can interact with the journal in a variety of ways:
 * Write in Journal
 * Put in Inventory
 * Put Away
 * Hide Journal: Sim takes journal to place in their bed.
 * Relive Memories: Gives random emotional moodlets.
 * Locate Journal: Once within a bed.
 * Snoop in Journal: Non-journal owners only.

Trivia
In the Sims 4 Parenthood, the black notebook setting is a reference to the anime "Death Note"