The "onset" is the initial phonological unit of any word (e.g. c in cat) and the term "rime" refers to the string of letters that follow, usually a vowel and final consonants (e.g. at in cat).
Watch this video which shows the manipulation of the initial sound. These activities are all about HEARING the differences in sounds in words.
Games
Play "Construct a word" by adding the onsets and rimes to make up your own words.
Take a look at this easy interactive onset/rime game to play:
Activities
Using the books below or others of similar format, complete the following activities:
Have your child find a set of rhyming words and write those down vertically. Then have them separate the rime portion of the word and write that ending several times below the first two words. Students will then choose initial sounds to place as the "onset" and create several new words within the same "word family".
Write down favorite words from the book, sort them based on their "rime".
While reading choose a word which is used within the rhymes and ask the student to manipulate the onset. For example: "Say pig, without the /p/" and "change the first sound in socks to /d/."