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Sunday, June 16, 2024

Ask Ms. Scientist: boiling pasta and Scantron machines

Dear Ms. Scientist,

Why does pasta boil over?

Ricky U.

Maybe it’s just me, but I eat pasta at least four times a week. It’s cheap, quick, easy and tastes delicious. One of the things I hate most about making my simple meal is when the pasta foams up and boils over the pot, creating a big mess all over my stove. To understand why pasta boils over, it’s important to know what pasta is made of. Pasta is essentially a giant starch with some protein; The key ingredients are flour, water and the occasional egg. When pasta is heated in a wet environment, the starch absorbs water and eventually bursts, sending tons of tiny starch molecules into the water and creating the white foam. The starch makes this foam stretchy and allows the bubbles to avoid burst for longer periods of time. If this foam layer isn’t stirred around the pot or scraped off the top, the steam underneath it will gather, overheating water in the bottom of the pot. This trapped steam will eventually cause the bubbles to expand and pop, breaking the foamy surface at the top of the pot and causing the water below to spillover rapidly. The trick of placing the wooden spoon across the top is scientifically proven to work. Since the spoon is thermodynamically unstable, it hits the spoon and bursts. This creates a break in the surface, allowing the bubble to break down.

Dear Ms. Scientist,

How do Scantron machines work?

Tom H.

Scantron machines have been around and used for standardized testing since the mid-1970s. Meaning, people have painstakingly been filling in bubbles with number two pencils for years. Early Scantron machines would shine a light through the paper and detect where it didn’t come through. Then why not write in pen? The graphite of a number two pencil is opaque. On paper, graphite is shiny black to reflected light but opaque to transmitted light. The phototubes had to sense a mark that was within the range of light wavelengths, which is mostly blue. In this case, the mark had to be something opaque. Scantrons cannot detect black or blue because blue inks do not absorb blue light, and black inks weren’t opaque enough. New Scantron machines, however, now search for the mark. Instead of looking for the space around the mark, they use reflected light to see the mark. Since these newer machines look at the paper from both sides, a number two pencil is not necessary. So go ahead and ditch those yellow, wooden sticks.

Ask Ms. Scientist is written by Julie Spitzer. If you have a burning science question you want her to answer, email it to

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