what does 8 and 9 have in common?

That is a good start anon :)

 

They are both equally distant from 8.5

They are both real numbers

They are both integers

They are both natural numbers 

They are both positive numbers

They are both factors of 72, 144,....... etc

They are both single digit (base 10)

They are both assumed to be base ten

When raised to the power of 0  they both become 1

When multiplied by 0 they both become 0

When multiplied by infinity they both become infinity

When they are subtracted from themself they both become 0

When divided by themselves they both become 1

 

there are a few - there are lots more :)

They share a common factor: 1

They have the same vowels in their spelling: e and i

Each has a factor which is itself a square: 4 & 9