What does it mean that existing habits are the best cues?

Attaching a new behavior to an established habit leverages existing automation.

Example: A software engineer wanted to start a daily gratitude practice but could never remember to do it. She tried setting a phone alarm for 7 PM, but she was in different contexts every evening and the alarm felt intrusive and disconnected. Then she noticed that she already had a rock-solid habit: every night, after brushing her teeth, she plugged in her phone on her nightstand. She wrote "After I plug in my phone, I will write one thing I am grateful for" on a sticky note and placed it on the nightstand next to a small notebook. Within ten days the gratitude entry happened automatically — her hand reached for the notebook the moment the charging cable clicked in, the same way it had once reached for the phone to scroll.

Try this: Identify five habits you already perform every single day without fail — brushing your teeth, making coffee, sitting down at your desk, locking the front door, putting on your seatbelt. For each one, note the precise ending moment: the last physical action that signals the habit is complete. Now choose one new behavior you have been trying to build. Select the existing habit whose ending moment best matches the context the new behavior needs — the right location, the right energy level, the right available time. Write the formula "After I [ending moment of existing habit], I will [new behavior]" and post it where the existing habit occurs. Run it for seven days and record whether the new behavior fires each time.