Many sounds in related languages can change based on the requirements of accent, speed of speech, presence of vowels, and so forth. One of the most common kinds of change is spirantization, in which a stopped sound like "b" is replaced by a sound that can be sustained, like "v".
In phonology, the study of relationships between sounds in linguistics, we recognize that many sounds are related physically to each other. One sound can turn into another sound when speaking quickly, or when it collides with a certain sound. These changes are the reason why cognate words in a family of languages may have small variations that can make cognates difficult to identify. We shall discuss a certain kind of change, spirantization, which affects many sounds.
Spirantization is the second step in softening (lenition) after affrication. So spirantization is a fancy word for a certain kind of softening. What we most want to get out of this is that a consonant's quality is often changed by its interactions with vowels. Essentially, we like to consider stop sounds like /p/, /t/ and /k/ as they get softer. Affrication, briefly, is when a stop and a fricative (a sound with moving air) get together: /ts/, /t?/, /pf/, and so forth.
After affrication comes spirantization, in which the stop sound is replaced by a fricative, which can be held indefinitely by blowing air. Plenty English speakers, such as this writer, have a problem closing their lips all the way when speaking, so "rubber" (a /b/ trapped between two vowels) comes out sounding like "ruvver" or "rummer." The change /b/ > /v/ is a classic case of spirantization caused by vowels taking the hardness out of a stop. Get it? So let's consider some really simple cognates of Germanic languages.
EN: half DE: halb SV: halv
In this case, we have voiceless labiodental fricative /f/ in English contrasting with voiced bilabial stop /b/ in German and voiced labiodental fricative /v/ in Swedish. If only we could color-code our words; then it would be extra clear that /f/ and /v/ are actually the respective voiced and voiceless versions of the same sound, and they are divided from /b/ only in that it closes the lips all the way, taking the teeth out of the articulation. Add to this stew the observed reality of Germanic final consonant devoicing, and we get all three words sounding basically identical. Now here's the thing: Old high German had halb, and old Norse had halfr, an acceptable variation. So spirantization as a distinguisher of dialects and related languages goes way back. In this case there's no vowel, but there is voiced /l/ before the final consonant. Voicing gives /l/ vowel quality.
Consider therefore the following:
EN: life DE: Leben SV/DK/NO: liv
EN: yellow DE: gelb SV: gul
In the case of "yellow," we can see that consonants articulated further back in the mouth also can change to related sounds. Here the /g/ sound found in Old High German gelawi has transformed through the influence of the front vowel /e/ and the voiced /l/ into a /j/. Try it. Say "gellov," over and over and see how fast you get tired and start softening /g/ into /ç/ and finally into /j/, softening increasingly frontwards to meet the vowel. "Yellow" is also interesting because it shows the consonant at the end, which is articulated with the lips (as explained above), giving out entirely into a simple lip-rounding of the vowel. Round those lips when you say that last vowel: yellow.
Perhaps English dry/drought and German trocken are even related through lenition of the /k/ in "trocken" to the /ç/ in Old English "drygnes" (itself a lention from /g/)? See our article on defunct English /x/, spelled "gh."
Finally, languages from different families, but that are neighbors with each other, also show spirantization's influence. Consider the Latin-borrowed word for writing:
ES: escribir DE: schreiben SV: skriva
Just as above, the /b/ in romance Spanish gets softened to fricative /v/ in Swedish, and the /k/ of the same gets softened to /?/ (esh) in German.
We'd love to bombard the reader with other examples, but we'll stick just to this one set of sounds. There are tons of other cognates waiting to be discovered by an astute student of languages, who should now be more aware of the relationships between sounds, and of how they influence each other to change through natural processes of speech such as spirantization. Make sure to check out our sources below, which are full of additional evidence.