Invisible Ink is an iMessage effect on your iPhone, found within the Messages app, that hides your text or photos until the recipient swipes or taps the message to reveal it; you apply it by typing your message, then long-pressing the Send (up arrow) button, selecting "Invisible Ink" from the bubble effects menu, and tapping Send again.
Steps
Applying heat to the paper with lemon juice should have made your secret message visible. Lemon juice is a relatively strong organic acid as it contains citric acid. When you write on the paper with lemon juice, the acid weakens the fibers within the paper, and it starts to decompose.
Text messages, unlike face-to-face conversations, are anything but ephemeral. They live on the recipient's phone forever, begging to be saved in your camera roll or shown to someone else. Invisible ink signals that you don't want whatever you said repeated, creating a sense of intimacy and code of conduct.
How to hide photos on iPhone or iPad
Triggering iMessage effects
To use these iMessage effects, you must create a message with another iPhone, iPad, or Mac user. Effects will still show animations on your end when texting your green-bubbled Android friends, but the recipients won't be able to see them themselves.
Using Invisible Ink
Messages automatically uses the following screen effects for specific text strings:
Just long-press the send icon and select “Invisible Ink.” Your message will be sent with a shimmering effect that keeps the content masked. . To reveal the message, simply tap on the bubble a few times.
Open the Photos app. Tap the Albums tab. Scroll to Utilities and select Hidden. Use Face ID or Touch ID to unlock the album (if enabled).
Invisible ink, also known as security ink or sympathetic ink, is a substance used for writing, which is invisible either on application or soon thereafter, and can later be made visible by some means, such as heat or ultraviolet light. Invisible ink is one form of steganography.
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iCloud exploitation
Your iCloud account is a gateway to all your synced data, including text messages. Someone can spy on your messages by: Logging into your iCloud account from another device to view message backups. Using a shared Apple ID (common in families) to receive your iMessages on their device.