About 70% of Americans live paycheck to paycheck, and about 70% of Americans can’t afford an unexpected $500 expense, according to a recent Bankrate survey.

Those are scary statistics. And while I’m not in those two categories, I’m not far beyond them—that is, I’m a single millennial living in expensive New York City, and I don’t have much of a cushion in my bank account, and while a $500 expense would not wipe me out, it would do serious damage. In other words, I need to start saving more money, as do most Americans.

Here at Yahoo Finance, July has been our “Ways to Save” month. All month, we’ve run stories and videos on saving strategies, and we’ve asked our readers how they save.

For my own efforts, I’ve been using three very different mobile apps, each of which can help you set aside some money. I’ve had varied results.

Acorns for investing in equities

Acorns has raised nearly $62 million in funding since it launched in 2012, and it is by far the best known of the three apps I used.

The app works by rounding up your spare change (that is, the difference between a transaction total and the next whole dollar amount) and investing it in a portfolio of funds and equities, commission-free. So: If you’ve set your roundup limit at 30 cents, then when you pay $22.70 at the grocery store, Acorns puts 30 cents into the investment portfolio. You don’t even feel it.

But allowing automatic roundups still makes me a little nervous (what if I use my credit card a lot in one month, and suddenly I see a high total has gone in to Acorns?) so I’ve limited Acorns thus far. After a couple months of using it, I have just over $30 in my account.

Acorns is credited with making the “roundup” app a popular model that other companies have followed.

Lawnmower for investing in digital currency

Lawnmower began as a roundup app, like Acorns—but for investing in bitcoin. The app, through a plug-in with leading bitcoin wallet Coinbase, would use your spare change to invest in bitcoin.

I’ve covered digital currency extensively, so I know more about it than the average layperson knows or cares to know. It’s certainly not for everyone. I’m not sure I’d recommend buying bitcoin to my parents. And yet: bitcoin is up 120% in the last year. It might just be a good speculative investment, even if you’re clueless about the technology and its significance. So I’ve tried Lawnmower—cautiously, as with Acorns.

Lawnmower recently changed its system, ditching the roundup structure for a model that buys bitcoin for you once each month at a set amount you choose. And its reason for doing so is interesting: “We viewed the spare change roundups as more of a limitation,” says CFO Alex Sunnarborg. “It’s a cool and quirky thing because it’s kind of mindless, I think that’s why people like it, because setting up a budget can be really hard for people. But if you want a serious investment platform, spare change is going to be inherently low-volume. To get higher than $20 of spare change in a month, you have to swipe your card a lot. So we wanted to be a more serious investment tool for your profile.”

Of course, Sunnarborg is right—roundups can only invest so much money in a month, since they take the spare change from your credit card transactions. But that’s what makes it appealing as a safe, slow investment vehicle. After about eight months of using Lawnmower, I’ve got $65 worth of bitcoin in my Lawnmower account. Soon, the app will add the option to invest in ether, the cryptocurrency of Ethereum.

Digit for setting aside cash

Digit is a friendly bot (that came around before the explosion in chat bots fueled by Facebook Messenger) that operates by text message. You connect your bank account and Digit analyzes your spending habits to automatically set aside a small amount of your money each day, small enough that you won’t notice it’s gone.

Digit is not an investment account, nor is it even really a savings account, because it does not accrue interest. The idea is strictly to give you a reserve stash for emergencies. Digit sends you friendly text messages like, “Daniel, here’s how much you’ve saved this month!” It also lets you know when a large deposit or withdrawal has been made. At any time, you can text Digit and ask to transfer some of the money in your Digit account back to your bank account.

I have found Digit to be the most helpful of these three apps, even though, in a way, it does the least. In nearly a year of using Digit, it has set aside more than $2,000 for me. And I appreciate that it doesn’t take more than I can spare. When I’m suddenly hurting near the end of a month and need to pay rent, I text Digit and ask for a couple hundred dollars back.

So, how do you save? Got any tech tools you like for setting aside cash for a rainy day? Let us know in the comments, or on social media using #WaystoSave.

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Daniel Roberts is a writer at Yahoo Finance, covering sports business and technology. Follow him on Twitter at @readDanwrite.

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