Why is this the answer?

The answer is pig because this is a really clever word puzzle that asks you to look for secret letter clues inside other words. It is like a fun spelling detective game. Let's break down the clues to find each letter. The first line of the riddle says, "My first is twice in apple but not once in tart." If you look at the word A-P-P-L-E, you can see the letter P used two times. The word T-A-R-T does not use the letter P at all. This means the first letter of our answer is P. The second clue helps us find the next letter: "My second is in liver but not in heart." When you spell the word L-I-V-E-R, the letter I is right there, but if you look at H-E-A-R-T, the letter I is missing. This tells us the second letter we need is I. Now for the last letter: "My third is in giant and also in ghost." The letter that shows up in both the word G-I-A-N-T and the word G-H-O-S-T is the letter G. So, the final letter to complete the answer is G. If you take all three letters you found P, I, and G, and put them together, they spell P-I-G. The last line of the riddle gives a perfect final hint to check your work. It says the "Whole I’m best when I am roast." A pig is a farm animal that people sometimes cook whole, or roast, for a big meal. This is a brilliant example of a letter riddle that tricks you into spelling the answer instead of just guessing.