5. LEARN A NEW LANGUAGE - Although this can be hard to believe, learning another language stimulates the brain and opens up all sorts of pathways. This is because there is a lot of listening, hearing and focusing involved.
4. SOLVE PUZZLES - Games and puzzles will gradually sharpen your brain and enhance your problem-solving abilities. They can also prevent or delay the onset of dementia later in life.
PICK A NEW HOBBY - Learning a new skill is also a great way to give your brain a workout. Creative skills especially, like music, dance, and visual art, exercise different parts of your brain and all have incredible benefits. Learning how to play an instrument is another way to exercise your brain.
2. TAKE A COOKING CLASS - Cooking uses a number of senses: smell, touch, sight, and taste, which all involve different parts of the brain. This will help sharpen your memory and awareness. Learning a number of different recipes will also help improve your memory.
1. WRITE THINGS DOWN - Writing things down by hand is the easiest way to exercise your brain. Writing can actually coordinate both sides of your brain, improving your brain’s function. It can also boost your cognitive skills. Furthermore, many psychologists believe that writing something by hand has a longer-lasting effect on memory. This works for both adults and kids. Writing by hand also activates the regions of the brain linked to reading. This improves the brain function as you are using more of your brain.
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source: viralventura
mynd44, lalasticlala, Dominique

I agree with writing things down. I still keep notes, diary, calendar etc.