Riddles and brain teasers

As you know, I love black stories. Hovewer, brain teasers and riddles can be very funny too. Let's try with some of them....

What is at the end of a rainbow?

Two mothers and two daughters went out to eat, everyone ate one burger, yet only three burgers were eaten in all. How is this possible?

A cowboy rides into town in Friday, stays for three days, then leaves on Friday. How did he do it?

What's full of holes but still holds water?