The story in three lines. Beto notices a clothes peg has two shapes. Margit draws a protein's two halves shut, with the join in pencil. Kemi's two photographs show it open on one day and shut around a partner on another, and Danya learns that two different results can both be true.

One thing to do at the table. Find things with a joint: a peg, tongs, a stapler, a laptop lid. Draw each one open and shut. Then find the one part that bends, and ask: what makes it close?

The grown-up layer. Proteins are not frozen sculptures. Many have parts joined by flexible hinges, and they switch shape when a partner binds. A prediction usually gives one shape; experiments under different conditions can capture several. When two careful measurements disagree, the disagreement can be the discovery. Here it shows that the protein moves.

For the ride home. Ask: why didn't Kemi throw one of the photographs away?