Ever wonder about line 23 on your annual Form 1040? In 2002 Congress created a temporary $250 “above the line” tax deduction to acknowledge teachers for the money they spend on classroom materials that their employers won’t.

Teachers spend several hundred dollars a year out of their own pockets; the provision gives them about 25 cents on the dollar (the marginal tax rate of most teachers) in the form of a tax refund – that is, about $62. The original provision applied only for 2002 and 2003, was estimated to cost $400 million, and was tucked in a $42 billion grab-bag of new and old tax cuts intended to stimulate an economy spooked by the 9/11 attacks.

Thus this token to our teachers became one of a growing list of “extenders,” temporary tax provisions that Congress, after a lot of teeth-gnashing, has extended for a year or two since the 1980s. The teeth-gnashing is part of the drama of (routinely) deciding to put the cost on a credit card. The last time our legislators went through this exercise, 12 months ago, the educator expenses deduction still had an estimated cost of $200 million, one of 62 one-year gifts to businesses and individuals with an estimated price tag of $42 billion.

Before it adjourns this week, Congress will pass a bill, the ‘‘Protecting Americans from Tax Hikes Act of 2015,’’ which would cut taxes net $622 billion over the next decade. As one of the bill’s 130-plus provisions, the educator expenses deduction would be made permanent, and the $250 limit would be adjusted for inflation. Eligible deductions would be expanded to professional development, in addition to classroom supplies. (Teachers already spend nearly four times the $250 limit, according to surveys cited by a pair of St. John’s University professors).

The number of tax-filing teachers claiming the deduction climbed from 2.8 million in its first year to 3.8 million in 2011, according to the St. John’s professors, and the cost of the deduction rose from $712 million to $962 million over the same period – far more than Congress estimated in either 2002 or 2015.

This is how Congress works. A legislator has an idea to spend money through the tax code. It gets enacted, often temporarily and almost always without an offsetting tax increase (it’s good to have your idea take off during a recession). Sometimes the idea is noble – who doesn’t want to support teachers? (Thus it’s cover for other ideas less noble with which it’s packaged.) Eventually the provision gains enough adherents and the time is opportune to make it permanent. Thus the spending goes on autopilot, without the bother of justifying it alone or compared to other priorities, or appropriating the money through the annual process. Nor is it subject to a cost-benefit analysis in an either big or small picture: Is this the best thing to spend money on, or the best way to support teachers?

What if we paid our teachers better? That’s not a concern of Congress, which just passed a big reform of federal support for K-12 education last week. But it feels good to give teachers $62 a year, especially since their students (who don’t yet vote) will be paying the bill – with interest.