Train Several Useful Skills

Variety keeps practice from getting stale, but it does not guarantee a broad upgrade to all thinking. The same puzzle every day mostly makes you better at that puzzle. A useful 10-minute stack touches several skills you actually use:

• Working memory holds and juggles information
• Processing speed thinks quickly under a little pressure
• Attention control stays locked on one thing
• Mental flexibility switches between ideas

A good stack covers more than your favorite drill, while staying honest: gains are strongest near the skills you practice.