NEW YORK (AP) — Millennials, it's time to pick up the phone and start saving.

That's the message from digital saving companies such as Acorns, Digit and Stash that cater to people under 35. They offer services for the smartphone that aim to be easy to use, have low fees and don't require large deposits. Stash, for example, lets users invest in the stock market for as little as $5. Digit and Acorns, meanwhile, do the work for savers by automatically transferring small amounts from a checking account to saving or investment accounts.

"I think that these apps are useful," says Paul Golden, a spokesman for the nonprofit National Endowment for Financial Education, because they teach people the habit of putting small amounts of money into savings.

The startups, all of which launched in the last year and a half, say they are targeting young people that have been ignored by traditional banks and brokerage firms. Many of them charge fees or require a minimum opening deposit that may be too high for some millennials.

"By and large big banks and brokerage firms are not focused on millennials," says Douglas A. Boneparth, a certified financial planner and partner at Life and Wealth Planning, which advises many millennials. "They are more focused on baby boomers that have assets today."

Saving money can be a challenge for millennials because many are saddled with student loan debt, says Boneparth. But he warns that those who want to use the apps still need to come up with financial plans and goals, like knowing what they are saving for and a plan to reach that goal, instead of just setting their savings on autopilot and forgetting it.

Here's a look at how some of the services work:

ACORNS

WHAT IT DOES: The app automatically puts money away for users by rounding up every credit or debit card purchase they make. For example, a $9.95 purchase at a grocery store will get savers 5 cents in an Acorns account.

HOW IT WORKS: Users link their credit or debit cards to the app for the round-up feature or can set up recurring automatic deposits to Acorns. Money is invested in a portfolio of exchange traded funds, which are a basket of stocks or bonds.

WHAT IT COSTS: $1 per month for balances below $5,000 and 0.25 percent per year for balances above $5,000.

CHASING MILLENNIALS: College students can use Acorns for free.

ONLINE: https://www.acorns.com

DIGIT

WHAT IT DOES: Digit links to a checking account and its algorithm looks at account activity to figure out how much you typically earn and spend. Based on that, it transfers small amounts into a Digit account.

HOW IT WORKS: Sign up is done on its website and Digit communicates with users through text messages. An app for Digit is in the works.

WHAT IT COSTS: Nothing, but since Digit communicates though text messages you may rack up texting fees.

CHASING MILLENNIALS: Digit tries to bring some fun to saving by sending users animated images, known as GIFs, when they reach a savings milestone. The first time money is deposited into a Digit account, for example, it sends a GIF of cartoon character Scrooge McDuck diving into a pile of gold coins.

ONLINE: https://digit.co

STASH

WHAT IT DOES: The app lets users invest in about 30 different ETFs of their choosing.

HOW IT WORKS: Users link their checking accounts to invest in the ETFs.

WHAT IT COSTS: $1 per month for balances below $5,000 or 0.25 percent per year for balances above $5,000.

CHASING MILLENNIALS: Stash tries to simplify the stock market for first-time investors. ETFs, for example, have been renamed. The Global X Social Media Index ETF is called Social Media Mania on Stash and invests in LinkedIn Corp., Facebook Inc. and other stocks. Stash also has a glossary throughout the app, explaining what a ticker symbol is or what historical performance means.

ONLINE: https://www.stashinvest.com

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Follow Joseph Pisani at http://twitter.com/josephpisani . His work can be found at http://bigstory.ap.org/author/joseph-pisani .