October may not be the cruelest month for investors. Based on the averages, that's September. But when Wall Street stumbles at this point of the year, it does so extra hard. And that's why, in the Oct. 3 episode of Motley Fool Answers, Alison Southwick and Robert Brokamp are joined by former Fool Morgan Housel to kick off a four-part series on the history of market crashes in the United States. In this episode and segment, they discuss the big one -- the Great Depression -- and a key lesson we should take away from it. What about the old adage of buy low, sell high? You need to have some cash on hand after markets tank to pull it off successfully.

A full transcript follows the video.

This video was recorded on Oct. 3, 2017.

Alison Southwick: So as we're winding down here, what is your takeaway for investors? What's one good lesson from the Great Depression that our listeners should take away?

Morgan Housel: There was a lawyer during the Great Depression named Benjamin Roth, who kept a really incredible diary. He was a lawyer, but he was also an amateur investor and an amateur economist. A really smart guy. And his son published a biography five years ago. It's called The Great Depression: A Diary, and it's really fascinating to see a layman's perception of what happened during the Depression. And he constantly writes about that in 1932, 1933 -- he uses the same phrasing over and over again -- "everyone knows stocks are cheap, but nobody has any cash to buy them."

He just talks about it all over the place. And not just stocks. He's talking about buildings and real estate in his neighborhood. There's a warehouse down the street. It's selling for nothing, but nobody has any cash to buy it. And he writes about it in the sense of all this opportunity that's lost, and if anyone had any cash during that period they could mint a fortune. There was opportunity laying right in front of them but no one had any cash saved up.

I used to write about this quite a bit when I was here at The Motley Fool. People really discount cash as an asset when things are going well. Cash doesn't earn a return. Why would you want to earn cash? Put your money to work. It's not doing anything for you. The value of cash is what it can do for you when things turn down, and things eventually will. That's when you earn your return on cash.

So I've always held more cash than I think any financial advisor would say is necessary, but that's why I do it, and I think I'm earning a good return on my cash. I'm just not going to realize that return until things get hairy again.

Southwick: So you're maybe hoping for a little downturn so you can put that cash to work.

Housel: No, not a great depression. Just a minor depression.

Southwick: That's OK. A so-so depression.

Housel: Just an OK depression.

Robert Brokamp: A so-so depression.

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