The equals sign is a balance

A bag of marbles goes in the left pan. Seven loose marbles, not in the bag, go in beside it. Twelve marbles go in the right pan.

We do not know how many marbles the bag holds. We write that number as the letter x.

Two pans, one claim

Call the bag's count x. The left pan holds x + 7. The right pan holds 12.

A level beam is flat. It says both pans hold the same number: x + 7 = 12.

A line like this is an equation. Each part beside the equals sign is a side. The number the letter hides is the unknown.

Rule: The beam is level only when both sides match.