The story of Bedřich Hrozný realizing that Hittite was Indo-European when he saw that wa-a-tar must mean ‘water’ is well loved among historical linguists and is familiar to anyone who's read anything about the decipherment of Hittite. But it's easy to misunderstand, and Piotr Gąsiorowski devotes his latest Language Evolution post to explaining exactly how the correlation works and why it is so important. Much has been learned since I left the field several decades ago, and I confess I find it both exciting and moving to see the correlations laid out so clearly and convincingly; it's one of those things that makes me intensely nostalgic for the days when I had my nose in dusty volumes of Kuhns Zeitschrift. Here's his conclusion:
To sum up, the fact that Hittite wātar is similar to English water is interesting but not particularly impressive as an isolated observation. Similarities can be found between any languages chosen at random. It’s far more significant that the inflectional pattern visible in Hittite helps us to understand the origin of the diversity displayed by cognate ‘water’ words elsewhere in the IE family and is part of the evidence used in the reconstruction of the PIE morphological system. It’s those pervasive shared patterns that demonstrate the membership of Hittite in the IE family.But I urge you to read the whole thing; it's a mini-course in Indo-European, and if you can get it under your belt you'll never again be fooled by flashy claims based on surface similarity.