The two mnemonics that work
Star Wars main theme. The first two notes ("Star" to "Wars" in your head) are a perfect 5th up. Twinkle Twinkle Little Star: same shape, slower tempo, the leap is the second word.
The piano sample above plays C and G. Most people can produce the sound without thinking once one of those mnemonics is in their head. The work is learning to name it on the way in.
Why this one matters
Two notes a perfect 5th apart vibrate in a 3:2 ratio. That's the second-simplest ratio in music after the octave's 2:1, which is why the perfect 5th feels so stable. It also explains why almost every Western scale can be derived by stacking perfect 5ths.
More practically: the perfect 5th is the outer span of every major and minor triad. C major is C–E–G. C minor is C–E♭–G. The 5th is the same in both. Once you can sing the 5th cold, you can find the rest of any triad by listening to what's between the root and that anchor.
Where you'll keep meeting it
Power chords are just a root and a fifth, played loud. The bass-line move from I to V (the basis of every blues, most pop choruses, half of Beethoven) is a perfect 5th up. Bagpipe and hurdy-gurdy drones hold the 5th underneath their melody to keep the harmony grounded. The whole violin family is tuned in perfect 5ths: violin is GDAE, viola and cello are CGDA. Mandolin uses the same intervals as the violin. Standard guitar, for what it's worth, is tuned in perfect 4ths with one major 3rd between the G and B strings.
What it gets confused with
Two intervals sound close enough to the 5th to cause trouble early on: the perfect 4th below it (five semitones) and the major 6th above (nine). The 4th feels like it wants to keep going somewhere. The 6th sounds yearning, like it's reaching. The 5th sounds like it has arrived and intends to stay.
A practical test: if you can sing a major triad starting from the second note of the interval, you've probably got a 5th in front of you. If humming the triad feels forced, it's something else.
How Aubel handles it
The perfect 5th is in the starting pool for every new player. The first morning alarm shows it alongside the unison, the perfect 4th, and the octave. The mnemonic the app surfaces on the detail screen is Twinkle Twinkle, because almost everyone can sing it on command. Star Wars sits there as a fallback for the people who happen to know that score better.