Planet 10 focuses on prefixes and suffixes. A prefix is a word beginning that adds to an already-complete root word and changes its meaning.
The prefix re means again, so to restart something is to start it again. But not all words that start with re have a prefix, such as the word reach.
The prefix un means the opposite of or not. To unlock something is the opposite of locking it.
The prefix pre means before. To preheat an oven is to heat it before beginning to cook something.