I confess my first reaction was 'Well, that's kinda pointless.' That was almost immediately followed by the realization that I had such a person in mind. With Gregor Mendel, I want to know if he cooked the books. Was his genetics research just incredibly lucky because he happened to pick seven traits, one from each of the seven different chromosomes (yeah, I'd have to explain chromosomes in the process) in the pea plants, or had he started with other traits, and thrown out the ones that didn't inherit independently of the others? As for telling him something, he might find it a comfort to know that while his monograph on genetics would languish for some fifty years, someone would later dust it off, and make something of if.