What do the following three-letter words have in common that has nothing to do with their meanings?

1. FAT
2. CAN
3. PEN
4. TON
5. CAB

HINT:
The same two letters will change each word into a new one.

Answers:

The letters AL can be added at the end of each clue word to make a new word:

1. FATAL
2. CANAL
3. PENAL
4. TONAL
5. CABAL