Someone covering your register so you can actually eat is friendship in its plainest form.

Kindness at work rarely looks like anything. It looks like a person absorbing twenty minutes of annoyance so you can sit down, and not mentioning it afterwards. Those are the favours that build a crew who cover for each other all year, and each one costs a single shift’s small inconvenience.

Do it back without being asked and without keeping score out loud. Notice who has not had a break yet, and tell them to go. If somebody did it for you lately, say thanks in words rather than assuming they know you noticed.