When we talk about letters and words, we know that

**A**is a letter,

and

**Apple**is a word with the letters A, p, p, l, e.

For numbers, instead of saying

**letters**we call them

**digits**So the number 174 has the digits 1, 7, 4.

The numbers we use every day go from 0 to 9.

The

**digits**are: 0, 1, 2, 3, 4, 5, 6, 7, 8, 9.

(The number 10 uses the

**digits**1 and 0.)

Since there are ten

**digits**, we use the word

**decimal**.

What if we only had two

**digits**to use, 0 and 1?

Could we still make numbers?

Yes!

We can make the numbers: 0, 1, 10, 11, 100, 101, 110, 111 and so on!

Since we're only using 2

**digits**, we use the word

**binary**(like bicycle which has two wheels)

When we want to match up our

**decimal**numbers to our

**binary**numbers,

we just put them in the same order.

**binary**0, 1, 10, 11 is the same as

**decimal**0, 1, 2, 3

So b0 = d0, b1 = d1, b2 = d10, b3 = d11 and so on!

We already learned about how computers use 1 and 0 to turn lights on and off on your computer screen.

The whole computer uses just

**binary**numbers to do all the math to turn those switches on and off.

Just think, giving a computer a number like: 1110 can be like telling it to turn 3 lights on, and the other one off!

(from: wikipedia - binary number)