Dropbox co-founder Drew Houston had a great Q & A in The New York Times. Inc. recently talked about the best leadership tips Houston had from driving the startup to unicorn status, but my favorite part is a personal gem dropped toward the end of the conversation.

I did the commencement speech for M.I.T. in 2013. I said that if I had a cheat sheet that I could give myself at 22, it would have three things on it: a tennis ball, a circle and the number 30,000.

The tennis ball is about finding the thing you're obsessed with. The most successful people and successful entrepreneurs I know are all obsessed with solving a problem that really matters to them. I use the tennis ball for that idea because of my dog, who gets this crazy, obsessed look on her face when you throw the ball for her.

The circle is really about the idea that you're the average of your five closest friends, so make sure to put yourself in an environment that pulls the best out of you. And the last is the number 30,000. When I was 24, I came across this website that says most people live for about 30,000 days. So you have to make every day count.

The tennis ball is clear--entrepreneurs have to be obsessives--as is the circle defining you. But 30,000 hours? It's brilliant.

Let's put it in perspective. What can you do in an hour?

Houston's well-served philosophy reminds me of Tim Urban's visualization of the amount of days we have left.

However, while Urban's is a sad reminder of how little time we have, Houston's perspective should give you hope: The hour you spend next could, and should, propel you toward your dream. How are you going to spend the next hour?