Perfect intervals (such as a fifth) tend to sound hollow and empty while intervals such as major third have more color to them. As for differentiating between the perfect fourth and perfect fifth, the perfect fourth still sounds hollow but has an element of dissonance absent from a perfect fifth, which sounds more pure. Keep playing the level every day and eventually you'll get it. Consistency over time is most important for mastering these exercises.

Thanks Thomas

if you have a piano it’s really helpful to try and figure out each interval (or just the root note) on your own first and then select an answer to see if you are right. You can also use the c key after you answer to pause the game with the notes in the interval displayed for you. you can then play both notes on your piano separately and together to get more used to picking out the root note. But thomas is absolutely right consistency over time is the most important thing. stick with it, i remember when consistently differentiating between a P4 and P5 seemed impossible to me

Hearing the 4th and 5th intervals was hard for me in the beginning. Also, they are inversions of each other. C is the 5th of F, and F is the 4th of C, so they can sound similar. Really focus on how each inversion makes you feel, or imagery in your mind. Close your eyes. Also, if you can find familiar reference songs that use those intervals in harmony.

it’s a distinct skill. i think being able to sing the notes played harmonically is key here.

I felt the same at first. Keep practicing and it will become second nature like other interval exercises 😉