In many Indo-European languages, some languages will use the same word but in varying forms that result from the alternation of two or more sound articulations that are physically related to each other.
The word we use in phonetics, the scientific study of what mouth articulations are in a given language, for the various /l/ and /r/ sounds is lateral. This is because we make the sound by blowing air around the sides of the tongue. Ironically, the word lateral contains both /l/ and /r/ sounds. Their phonetic "feature" is called liquid. The lateral is associated physically with the tongue tap /d/.
An astute student of languages will notice that, between languages that share a word, such as one descended from Latin or old Germanic languages, the word will contain the same sounds in both languages, but will be arranged differently in the word. For example students of English and Spanish will notice that laterals are often alternated.This is possible through the rules of syllable structure for each language, and many European languages' syllable rules accomodate those of others. Also, in many cases, the sounds that alternate are so similar, when looked at from a purely physical point of view, that they alternate naturally. Remember that, in English, young children who haven't mastered /r/ yet will often pronounce /r/ as /l/.
There are tons of examples of l/r alternation, so we will use some that compare seemingly different languages that are in fact closely related.
ES milagro = EN miracle. Notice that the second consonant is /l/ in one, /r/ in the other, and the other way around in the fourth consonant. One may also notice that the letters g and c mark the same sound, a velar stop, /g/ being voiced and /k/ being unvoiced.
PT obrigado = ES obligado = EN obliged. The same case as above.
DE Karzer = ES cárcel. The r sound in High German is of course not a lateral rhotic, but a velar like in French, however in plenty of dialects it's still done with the tongue. The word Karzer is in fact borrowed from Latin, in the same way that English uses incarcerate. Cárcel is the modern Spanish word for "jail."
DE Maser = EN Measle. Here is a more clear-cut example of two languages that have the same ancestor varying l/r syllable-end sounds.
Finally, here is another simple one:
SV Prata infinitive, declined in the present as pratar = EN slang prattle.
Try it! Look for cognates between the languages you study that demonstrate alternation. L/R is just one of them, and there are tons of examples of just that one.