Two decades ago Arizona voters said teachers weren't being paid enough and they did something about it, approving a new sales tax for schools.

Today, Arizona voters are saying the same thing, and Gov. Doug Ducey is now getting behind an extension of that sales tax.

The sales tax was part of Proposition 301, which is set to expire in just four years. Educators warn that deadline is closer than it looks.

Here are five things to know about Prop 301:

How it works

We pay an Arizona teacher whenever we go shopping.

For every $10 you spend, 6 cents in sales taxes are dedicated to education. Those pennies add up to $600 million a year.

Most of those pennies, about 4 cents, pay for K-12 teacher and staff salaries -- almost $400 million a year.

By some estimates, the money could account for up to $10,000 of the highest-paid teachers' salaries.

Who approved it

Low teacher pay was a big issue in 2000, when Arizona voters approved the new six-tenths of a penny tax in Prop 301.

"It's our only hope for raising teacher salaries and better supporting our kids," one editorial writer said at the time. Sound familiar?

The average Arizona teacher salary then was $35,025 -- 34th in the country.

Seventeen years later, the average teacher pay is $46,384, now among the lowest in the country.

Despite the growth over the years, Arizona teacher pay hasn't kept up with inflation.

Why it's in trouble

Prop 301 is barreling toward a fiscal cliff -- and taking teacher salaries with it. The tax expires in 2021, just four years away. Voters statewide would have to renew it.

What the governor says: The governor's support is crucial to renewing the tax. Gov. Doug Ducey is now saying he backs an extension of the sales tax for schools. But the anti-tax governor, who has promised to cut taxes every year he's in office, doesn't want to call it a tax. It's a "funding program."

What happens next?

Educators, lawmakers and the governor would have to update Prop 301 and refer it to a statewide vote.

Ducey says he wants a "modern" Prop 301. We don't know what that means yet.

But if all sides come together, a statewide vote on extending Prop 301 could happen as soon as November 2018.

The question is whether Ducey would want to campaign for re-election next year while pitching what some Republicans would label a tax increase.

Educators warn that school districts are already preparing for a phase-out of the tax.

Delaying a vote unil the next presidential election in November 2020, as some want to do, would push school district budgets perilously close to the fiscal cliff.