Decide whether or not the following experiment is internally valid. If not identify the source of threat to its internally validity. Professor Beach wanted to demonstrate to his cognitive psychology class how using retrieval cues can improve memory. First, he read a list of 16 words in random order to his class, and immediately afterwards he had them write down all the words they could remember. On average, they recalled about seven words. Then, he told them that the words could be sorted into four categories: automobiles, cutting instruments, sports, and fruits. Professor Beach encouraged his students to do this in their minds while listening to the list again, after which they would be given another chance to recall the words. On the second test of recall, students remembered, on average, 14 words. Professor Beach told his students that this proves the effectiveness of using retrieval cues as a mnemonic strategy.