mass ball pack pat
These four words share an unusual property (and there are at least 9 others that do as well, but I had to use wiktionary to confirm some of them).
fand is not a word, but if it were a word, it would also share this property.
When you figure it out, think up two more words that share this property, and send me the answers.
Several of you have noted that the 'a' in these words is a short vowel. The property has nothing to do with how the words are pronounced (directly, anyway), but it does matter how they are spelled.
You've almost got it!