Have you ever seen little kids swimming even after their lips turn blue and their teeth are chattering? They eventually come out of the water, but long after rational adults would.

Welcome to the U.S. stock market. Stock valuations are the highest they have been since the tech bubble, with valuations rising 25 percent over the last three years, yet the kids aren't even making waves.

In fact, the Dow rose every single month in 2017 and the MSCI All Country World Index has not dropped more than 5 percent for the longest period since its inception.

When kids got cold swimming at a public pool, they would race into the warmth of the hot tub before plunging back to play. International stocks, which by almost every measure are less expensive than U.S. stocks, may be that hot tub. But you can't stay in there forever, either.

So is it time to grab a towel and get out of the market? The problem with trying to time the broad market is that expensive stocks can get more expensive for a while. Getting too conservative too early can be as costly as being too aggressive too late.

It's time to get out of stocks if you know you are going to be needing the money in the next two to three years. This means if you have direct expenses coming up that are not likely to be delayed — college costs, cash to replace earnings when you retire, charity or other short-term expenses — sell stocks to set aside money for this.

Investing is rewarding over longer periods of time because of time and compound interest (your interest earning interest). If you need the money quickly, you have neither of those working for you. You have moved from investing to gambling. Then I can promise you that if the markets continue to advance, you will come up with reasons why you can keep your short-term money invested.

But even if it ends up working for you, short-term investing was simply a bad decision with a good result. Investing when you really need the cash is like swimming during lightning. You might not get struck if you keep swimming, but why risk it?

If you are invested for the long-term, though, it doesn't mean you should never make changes. Make sure that your investments are in a variety of categories — large and small stocks, U.S. and foreign stocks, and bonds.

Bonds look like a lousy investment in this current environment of potentially rising interest rates, but you should be using them defensively — short term, safe bonds that you will sell and reinvest in stocks if the market falls. Determine the allocation ratio with which you feel comfortable and rebalance back to that. You can even look at some asset allocation funds (not target date funds) that will automatically do this for you. Don't make a complex investing decision be one extreme or the other. Aim for moderation.

If you have come into cash from an inheritance, bonus, or sale of a business, don't simply jump into the pool. Gradually get your feet wet by setting up a periodic investment plan. We like to think about date and price triggers for investing.

For example, invest a certain percentage of your cash every four weeks and every time the market falls by 3 percent. Date triggers mean that you won't miss investing if the market doesn't correct and price triggers mean that you will get better pricing if the market falls or bounces around. This allows you to test and gradually get used to the water rather than experience the shock of jumping in.

If your life expectancy is compromised and you have stock outside of your retirement plans that has appreciated a great deal, you should rarely sell it. If you die with an asset that has grown in value, your heirs get a step-up in basis. This means that they can sell that stock and not pay capital gains.

Depending on your capital gains tax rate, you shouldn't incur the certainty of paying that tax for the uncertainty of a market correction. If you are charitably inclined, though, you could donate that appreciated stock and get the full deduction for the value of the gift and avoid capital gains taxes.

If you hold the pool controls and want to stay swimming, you may be tempted to keep turning up the heat until it bubbles. But we are far from a bubble in this market. A bubble is often indicated by rapidly accelerating prices (bitcoin, anyone?). While this market is expensive, it has been a pretty orderly ascent. Remember, though, the apocryphal story of the frog that gets boiled by the heat gradually rising over time. Stay on your plan and you won't get cooked.

Some of the things that have helped this market — deregulation, lower corporate taxes, accommodating central banks — are most likely factored into current stock prices. Think of these as more exotic pool toys that children initially enjoy playing with but eventually tire of. We are nearing that point.

Ross Levin is the chief executive officer & founder of Accredited Investors Wealth Management in Edina