You fill an ice cube tray with water and put it in the freezer.
It takes a little while -- but, if you don't open the door and let the warm air in -- you eventually get ice.
That same process was at work on Lake Erie this week.
Since last Saturday's unseasonably warm 43 degrees, temperatures dropped below the freezing point Sunday and remained there all week.
And the lake basin went from mostly unfrozen a week ago to more than 27 percent covered in ice by midnight Friday, according to NOAA's Great Lakes Environmental Research Laboratory.
The ice coverage on Lake Erie is still well below-average for this calendar date. It's usually about 47.3% frozen over by Jan. 23, according to NOAA data.
And, when compared to the past two years, it's incredibly wide open with water.
The temperature on Lake Erie has slid precipitously over the first three weeks of the New Year. It hit 32 degrees at Buffalo Harbor on Wednesday after starting the New Year at 41 degrees, according to the National Weather Service.
Ice on the eastern end of the lake is mostly visible only along the immediate shorelines.
But, with the cold week, and increased ice coverage, fishermen also began showing up again in protected places like the Small Boat Harbor.
Whether Lake Erie will continue its march toward more ice coverage will be dependent on temperatures and wind speeds over the next week or so.
Forecasts show the temperature will remain below freezing until Monday but rain is forecast on Tuesday. And, the thermometer will get back to the low 40s.
Early forecasts for February show a trend toward warmer conditions as well.
NOAA climate experts forecast the Buffalo Niagara region could be 8 degrees or warmer than average during February, and forecasters from AccuWeather predict that nearly half of the month's 29 days will reach or exceed 40 degrees.