The suffix -ful attaches to the end of words to mean “full of” or “having the quality of.” Add it to care and you get careful, meaning full of care or paying close attention. Add it to joy and you get joyful, meaning full of joy or very happy.

This little suffix transforms nouns into adjectives that describe what something is like. A powerful engine has a lot of power. A peaceful morning has the quality of peace. A colorful painting has many colors. The pattern works across hundreds of words: hopeful, grateful, beautiful, wonderful, faithful, playful, and countless others.

The suffix can also turn words into nouns that describe amounts. A handful is as much as you can hold in your hand. A cupful is the amount that fills a cup. A mouthful is what fills your mouth, whether that's food or perhaps too many words at once.

Sometimes people misspell words with this suffix by adding two l's at the end, writing “carefull” or “peacefull.” But remember: when -ful becomes part of another word, it keeps just one l, even though the word full on its own has two.