The number 5 is the third prime in the sequence. It is the only prime number to end with the digit 5 (all other primes apart from 2 end in 1, 3, 7 and 9) and so it remains unique among the odd numbers. It is also the only number that is the twin of two primes (both 3 and 7 are its twin.) There are only 5 Platonic solids.