"Buy one, get one free!" Remember when Bill Clinton said that about his wife, Hillary, when he was running for his first term back in 1992?

Two bad ideas for the price of one, right?

Now it appears that Hillary Clinton herself, running for her own presidency has picked up her hubster's meme, endorsing Philadelphia Democratic Mayor Jim Kenney's proposal to slap a 3-cent tax onto bottles of soda pop to pay for universal pre-school for Philadelphians.

Two bad ideas for the price of one.

The former secretary of state was advocating for a “cradle to college” pipeline when the soda tax proposal came up during a gun control forum hosted by the Mothers of the Movement in Philadelphia.

“It starts early with working with families, working with kids, building up community resources,” Mrs. Clinton said. “I’m very supportive of the mayor’s proposal to tax soda to get universal preschool for kids.”

“I mean, we need universal preschool. And if that’s a way to do it, that’s how we should do it,” she added.

Actually three bad ideas for the price of one: gun control!

But I'm going to skip that third one in honor of the Obama administration's selection of rifle-and-pistol-toting Harriet Tubman for the $20 bill.

But as for the other two…

Now, I'm the daughter of a mom who refused to allow a carbonated soft drink to cross the threshold of our family home. "It rots your teeth" was her mantra. And to this day I drink soda pop–classic Coke or classic ginger ale–only as a beverage of last resort. But the currently fashionable demonization of sugar alone of all foods as the cause of all human health and social ills is silly and unscientific. As Alan Levinovitz wrote last year for Real Clear Science:

Some researchers, including Robert Lustig, a pediatric endocrinologist from the University of California San Francisco, have described sugar as toxic. While we know that Americans are consuming a lot more sugar now than they have in the past, we don’t know at what amount it goes from being a sweet treat to something dangerous. Nor do we know how much of that danger is due to sugar’s unique effects, as opposed to its being a contributor to excessive caloric intake.

As for universal preschool, a wonk-fad already implemented in Democratic Mayor Bill De Blasio's New York City, there's still no evidence that sending 3- and 4-year-olds to school has any susbstantial or lasting effects on their educational outcomes later on. Grover J. "Russ" Whitehouse, writing for the Brookings Institute, points out that there simply hasn't been enough testing to discern which kind of pre-K programs work, or whether any of them work well enough across the board to merit universal application:

The Georgia study finds impacts that are at best very small and do not pass a cost-benefit test. The Texas study provides evidence for value in a targeted program and is silent on the effectiveness of a universal program. The Tulsa study and other studies that use a design that compares children who just meet or just miss the age cut-off for pre-K can't estimate the impact of state pre-K because they are comparing children that may differ in many experiences in addition to their participation in state pre-K.

This thin empirical gruel will not satisfy policymakers who want to practice evidence-based education. Their only recourse if they have to act is to do so cautiously and with the awareness that they are going to make some mistakes and need to be in a position to learn from them. They and the general public need to be wary of the prevailing wisdom that almost any investment in enhancing access to preschool is worthwhile. Some programs work for some children under some conditions. But, ah me, which programs, children, and conditions?

The moral to be drawn: When someone, especially if that someone is a Clinton, pitches two ideas for the price of one, be wary of both of them.