What goes up but never comes down?

You see a boat filled with people, yet there isn’t a single person on board. How is that possible?

If you go to the movies and you're paying, is it cheaper to take one friend to the movies twice, or two friends to the movies at the same time?

Christina has four daughters, and each of her daughters has a brother. How many children does Christina have?

Considering the below equation:

V + II - VI = 10
IV - X + VIII = 20
IV - VII + VI = 30

Then, I + II + III = ?

A certain street contains 100 buildings. They are numbered from 1 to 100. How many 9's are used in these numbers?

Replace the question mark with the correct number.

Using eight eights and addition only, can you make 1000?

How many cubic feet of dirt are in a hole of one foot deep, three feet long, and two feet wide?

The doctor advised his patient to take a tablet after every fifteen minutes. He gave him five tablets in total.

How much time will he take to have all the five tablets?