An empirical puzzle regarding toddlers' fast mapping motivated the current investigation. Whereas children between 1;10 and 2;1 have shown only a modest rate of mapping novel nouns onto unfamiliar rather than familiar objects, a very high rate has been observed in those between 1;4 and 1;8 (Mervis & Bertrand, 1994). Study 1 examined whether young two-year-olds (N=40, mean age=2;1) might map at a higher rate when tested with procedures unique to Mervis & Bertrand's assessment – strong corrective feedback rather than mild positive non-contingent feedback; large sets of test objects rather than pairs; presentation of easier tests first. Only the first variable affected performance in a manner that could solve the puzzle. Unfamiliar kinds were selected at a much higher rate under corrective (0·86) than non-contingent (0·57) feedback. Although nearly every child in the non-contingent group chose correctly on the first trial, many failed to do so thereafter. In Study 2, rather than presenting a test word to the children (N=16, mean age=2;2), the experimenter merely asked for ‘the one I want’. Unfamiliar kinds were selected much less often than in Study 1, suggesting that at least one lexical principle proposed in the literature underlies the noun mapping preference. Changes over trials in the two studies indicated that the noun mapping preference is quite prevalent, but unless initial choices are strongly reinforced, an increase in the salience of familiar kinds after the first trial lures some children into error. Consistent with this analysis, toddlers in Study 3 (N=24, mean age=2;1) who received non-contingent strong acceptance for their noun mapping decisions, selected unfamiliar kinds more often than those who had received non-contingent mild acceptance in Study 1.