A real number is normal in a base if its expansion does not favour any digit or string of digits. So, to be normal in binary means that, on average, there are as many 0s as 1s in the expansion, the string 101 appears one-eighth of the time, etc. A number is normal if it is normal in all bases. It is relatively straightforward to show that almost all numbers are normal but can be difficult to show a specific number is normal; it is not known whether π‎ or √2 is normal. Rational numbers are not normal.