What a great topic, I've been loving reading everyone else's responses to this. I'm discovering I know how to do a lot more things than I originally thought and there are definitely some things that I should know how to do at this point in my life and I'm still clueless. So here are my three things that I think are essential tools to have in your mental tool box:
1. Learn CPR and basic life support skills
You can literally save someone's life! Whether you are at home or out in public, you should know how to help if you witness a cardiac arrest. You don't need to be a medical professional to help, I can't stress that enough. It will take the ambulance a few minutes to get to the scene but a passerby-er performing CPR in those few minutes can make a lifetime of difference. More info here: redcross.org/take-a-class/cpr
2. Manage your finances!
Well here's the pot calling the kettle black as I struggle with this literally daily. I make just enough to pay my rent and other bills but there is nothing left over and I have no savings to speak of. Everyone should know how to save money and to make a budget and stick to it. If you have a job that offers a 401k (and hopefully they will match your contributions), take it! Start saving for retirement now! The sooner the better. Starting at 25 will be so much better than 35!
3. Know how to be respectful
Whether it's being a great house guest or holding a conversation with your mom's friend, just be polite! Say please and excuse me, send a thank you card to your grandma for the $50 she slipped you, don't trash the hotel room you're staying in, bring your cart back to the corral after you are finished grocery shopping. I'm sure different people have different definitions of respect, but to me everything I stated above are my definitions. Being respectful literally costs you nothing and it means everything.
here's an interesting video I found on Facebook, a little food for thought about this topic!

