math fun with the numbers 0 to 9

0 1 2 3 4 5 6 7 8 9
you need to add an odd and an even number to get another odd number

even numbers can only be made by adding 2 even numbers together

0+1+2+3+4+5+6+7+8+9=45
the sum is the same as the 2 middle numbers 4 and 5

every odd number is also equal to the 2 middle numbers between it and 0, e.g.:
1+2=3
2+3=5
3+4=7

the sum of every 2 outer numbers is also equal to the sum of the next inner pair of outer numbers, e.g.:
0+9 == 1+8 == 2+7 == 3+6 == 4+5

this also works no matter where you start the first pair of outer numbers, e.g.:
0+8 == 1+7 == 2+6 == 3+5 == 4(+ itself, 4)

sources:
Numerology Facts & Trivial facts of all ten Arabic numerals?