It is always ahead of me yet I can never see it. What is it?

I organized a small get together at my home.
In the party , i have a barrel with some whiskey in it.

Suddenly Guest 1 say 'I bet this barrel of whiskey is more than half full'.
'No, it's less than half full' Guest 2 replied.

I don't have any measuring instrument and without removing whiskey from it , how can i determine which of the guest is right ?

Accidentally, two trains are running in the opposite direction and enter a tunnel that is 200 miles long. A supersonic bird that has fled the lab and taken shelter in the tunnel starts flying from one train towards the other at a speed of 1000 mph. As soon as it reaches the second train, he starts flying back to avoid collision and meets the first train again at the other end. The bird keeps flying to and fro till the trains collide with each other.

What is the total distance that the supersonic bird has traveled till the trains collided?

He runs but never walks. Murmurs, but never talks. Has a bed, but never sleeps. And has a mouth, but never eats?

Can you count the number of blocks in the picture below?

What does below science rebus mean?

Ag Ag Ag
Ag SPOON Ag
Ag Ag Ag

What does this mathematical rebus means ?

what one word does the below rebus identifies?

If you had a pizza with crust thickness 'a' and radius 'z', what's the volume of the pizza?

What does a man do only once in his lifetime, but women do once a year after they are 29?