Think about a backyard garden.  If you walk to the garden from your back door, your footprints will mash the grass down for a few minutes, but the next day, those footprints seem to disappear.

But if you walk on the same path many times a day, every day for a whole year, those footsteps will turn into a path that will be clearly visible, and will stay there for a long time.

Your memory works the same way as that grass.  When we learn something new, our brain uses our short-term memory to complete the task.  By revisiting that same skill later, our brain strengthens the neural pathway (electrical connection) it built the first time, when you created the new memory.

Yes I know this, but they make you do stuff you have already mastered all the time...

So I came back 7h later and after 2 Master Challenge it says "No mastery challenges available. Practice something new!" I CANT!

I'm sorry you're having trouble with the timers, Gabrielle. Could you check if there are any "Needs practice" skills left on the grid? Here's how to find it:

https://khanacademy.zendesk.com/hc/en-us/articles/202875310-How-do-I-get-to-the-exercise-grid-in-the-new-dashboard-

If you see a grey tile, click it to practice and hopefully master it soon after. Let me know if this works for you. =)

I know I'm necroposting, but I'm having the same problem. Also, no offense, but any of us who can read can click the button that says "show all skills". Please help me, I want to get 7th grade done today.

Thanks!

@Ezzy
Are there any grey unpracticed skills left on the grid? Try refreshing the page a couple times.

No... I guess I’ll wait till tomorrow.

I do about 7 mastery challenges per day and then it tells me that "no mastery challenges are available." Sometimes although I have about 5-10 skills practiced, level one, or level two, it doesn't give me an opportunity to master it. Sometimes on mastery challenges on the Precalculus Mission I receive division problems. Why? We need to learn new math. Not keep iterating over and over on basic arithmetic.