'Fixer Upper' stars Chip and Joanna Gaines on the secret to their marriage

"Fixer Upper" stars Chip and Joanna Gaines had a big reason to celebrate recently. The parents of four children have been married for 13 years.

The secret to keeping the spark alive?

"One of the secrets to our marriage, or the success of our marriage, is just really caring about one another," Chip Gaines told ET.

The HGTV stars detailed their love story in their new book "The Magnolia Story" including the tale of Chip's proposal.

Chip brought his then-girlfriend Joanna to a family friend's jewelry store so they could design her engagement ring together. Now that the pair have had a lot of success, they contemplated replacing her diamond with a bigger stone but Joanna decided against it.

"This is the original diamond I picked, and it's perfect just the way it is," Joanna shared.

13 years and four kids later, the pair still have their disagreements. Like Joanna's wish to have two more kids.

"You wish," Chip told her.