In the 1880s a German scientist, Hermann Ebbinghaus, tested his own memory with great care. He learned lists of made-up syllables like DAX and BOK, then measured how much he had forgotten after twenty minutes, an hour, a day, a week and a month. He published his results in 1885.
He found a clear shape. Forgetting is fast at first, then it slows down. Much of what he forgot was gone within the first day; after that, what remained faded slowly.
Key idea
The forgetting curve drops steeply right after learning and then flattens. That means the first day or two matter most.
He also found something hopeful. Each time he relearned a list, it was forgotten more slowly the next time. Reviews at the right time reset the curve and make it flatter.
Why it works
Ebbinghaus used nonsense syllables on purpose, so that meaning would not help some items more than others. Meaningful material is forgotten more slowly, but the shape of the curve is similar.
Common mistake
Reading the curve as exact for everyone. It is a shape, not a law with fixed numbers. How fast you forget depends on the material, how well you learned it and how you review.