Birds and bees

"Answer this question, but honestly Mum
Why is it bees and humming birds hum"
 

"I know why they do it, yes I have the knowledge
Because after all I did go to college
 I’ve read every book i've and filled up my brain
The answers come quick like a runaway train
 The reason for humming of bees and of birds

Is because they can never remember the words"