Here it's a nice and easy to understand article about that:
Hope it helps
Thank you! but to be more specific, I am stuck at
I see, it seens to be a typo error: try to rename the variable 'myStrenght' to 'strength'
It's not a typo error, it's because lesson validation expects
strength, but you used
myStrength. So, your code is fine, but you need to do exectly what instructions says.