Rosenthal and Jacobson gave an intelligence test to all of the students at an elementary school at the beginning of the school year. Then, they randomly selected 20 percent of the students - without any relation to their test results - and reported to the teachers that these 20% of 'average' students were showing "unusual potential for intellectual growth" and could be expected to "bloom" in their academic performance by the end of the year. Eight months later, at the end of the academic year, they came back and re-tested all the students. Those labeled as "intelligent" children showed significantly greater increase in the new tests than the other children who were not singled out for the teachers' attention. This means that "the change in the teachers' expectations regarding the intellectual performance of these allegedly 'special' children had led to an actual change in the intellectual performance of these randomly selected children"
My understanding is that a person's IQ doesn't change. See Wikipedia
How is this result possible?