Twenty-five years later, we can easily replicate the whole setting, simulating this aha!-insight effect (inspect therefore particularly Figure 1(a) and (g)): Just take two sheets of paper (e.g., A4); one original-sized, one halved by folding, and compare them in terms of area size by centering the halved sheet on the center of the original one! We perceive the larger sheet as far less than double the size of the smaller one. For instance, by asking people to assess how much bigger the larger sheet is compared with the smaller one (in percentage; thus, “100%” would be the correct answer1); most people strongly underestimate the size of the larger sheet. When I asked 102 participants (undergraduates of psychology, 76 female, mean age 20.8 years), two main results were retrieved: (a) although the entrance requirements for starting the study of psychology are extremely high in Germany, 14 persons were not able to operate with percentages in a meaningful way, an inability which is not at all uncommon (Siegler & Lortie-Forgues, 2015) besides severe forms of problems with numbers and problems applying mathematical routines such as dyscalculia (with a prevalence rate of about 6%, see Shalev, Manor, Auerbach, & Gross-Tsur, 1998)—consequently these persons were excluded from further analyses; and (b) remaining participants (88 undergraduates of psychology, 68 female, mean age 20.9 years) showed the same aha-effect as was described in the photographic shop after having had rotated the sheets. Before doing this, I confronted them with a series of geometrical settings (see the top row of Figure 1 for the A4 settings), first consisting of an A4 and a halved A4 (i.e., A5) sheet, always starting with the setting shown on the very left side, progressing to the version on the very right side, one after another. After this A4 series, the same general settings were shown for the U.S.-letter size plus the halved U.S. letter (“half letter”), see the bottom row of Figure 1. Participants assessed the configurations of the first “A4” series very similarly to the second “letter” series—exact values plus effect sizes for one-sample t-tests against the true value of 100% can be retrieved from Figure 1. Figure 1.Overview of the employed experimental conditions, already in the experimental order which was realized, starting with a centered version (a/g) and always ending with a fully aligned version (f/h)—the participants were first exposed to the A4 paper size setting (Series #1), then to the U.S. letter size setting (Series #2). Percentage values show the mean estimations of how much bigger the larger sheet is compared with the smaller one (100% would be the correct answer, e.g., 64.0% in the case of Figure 1(a) means that the area of the bigger sheet is strongly underestimated, d = 1.16); *** indicate p-values < .001. Effect sizes are expressed as (Cohen’s) d’s for one-sample t-tests against 100%.