When writing music, it is necessary to indicate the relative duration of each note. The duration of a note is indicated by its appearance.

Happy Note! Notes in Space is a free computer game that makes it easy to learn and remember what shape of note corresponds to what relative duration (whole, half, quarter) as well as the ratio of any note duration to any other (1 whole note = 2 half notes; 1 half note = 2 quarter notes; 1 whole note = 4 quarter notes) :

For you to score, the duration indicated by the spaceship (whole, half, quarter) must match the duration of the note you're shooting at.
When you hit a whole note, it splits into 2 half notes; when you hit a half note, it splits into 2 quarter notes
When 2 quarter notes collide with each other, they turn into 1 half note. When 2 half notes collide with each other, they turn into 1 whole note.
At the start of each game, you must answer a multiple choice mini-quiz on note durations. Your answer will determine how many lives you start the game with.
You have the option to have the game display or not the duration of each note you hit.
You can configure Happy Note! Notes in Spaces so it will be suitable for players of any age:

There are three levels of difficulty: BEGINNER- ADVANCED- EXPERT
You can give yourself bonus points at the start of each game by clicking on the score indicator
You can activate or deactivate the comets and meteors at the start of each game