Observe what's happening with the sounds in these words: pay attention to the first sound.
| fire, German feuer | Greek pyr- |
| father, German vater | Grk patr-, Latin pater |
| feather, German Feder |
Grk peto, pt- |
| foot, German fuß | Grk pod-, Lat. ped- |
| fee, Old Norse fe | Lat. pecu- |
| five, German funf | Grk pent-, Lat. quint- |
| flex, fold, German falten |
Lat. plex, ply, pleat, plait, -ple |
| fathom | Lat. pateo, Grk. petalos (>>petal) |
What do you note about the initial sound of the words in each column?
Do the other sounds match, roughly enough, so that you could buy the claim that all the words in each row descend from the same etymon?
Do the other word's meanings match, roughly enough, so that you could buy the claim that all the words in each row descend from the same etymon?
Now, expand that out beyond German and English to many Germanic languages and many other Indo-European languages, and you will see the data set that Rask saw that led him to formulat the "law" that goes by the name of Grimm's Law.