"Why do Computers use Binary"

3 new things learned:

1. Phone encodes sound waves as a voltage signal and the headphones decode the voltage signals as sound waves to get sound to your ear. 
2. All computers bases on a base two system.  
3. They use base two binary  because it has the fewest digits, but binary numbers are a lot longer so it takes a longer time to send the numbers into the adder.  

2 new things you don't understand:

1. I don't understand how higher and lower voltages and pressures can create sound without anything else too it.
2. I don't understand why they would use digital and analog signals if they already use the binary system.  

1 thing you would like to know:

1. Do things such sound in an iPhone also use the binary system or just voltage signals?