I went to the public library, in the huge hall related to Experimental Physics , and found a method that will make you figure it out.
a) walk in an helix pattern until you will see these two items nearby:
-- a sleeping lion
-- a tree with low branches
You will surely succeed in this if you keep walking long enough.
b) suddenly pull the lion's tail with all your strength and even more.
Due to the complex structure of the Solar System, and the
Earth's gravity field, one of three events will always happen:
c1) the lion asks you for a cup of coffee and starts telling you in a soft voice about what it was dreaming of.
c2) the lion is so terribly scared seeing your face that its runs away
and you will never meet it again.
c3) the lion makes a thundering 'roar' and aims at taking you apart;
you will barely escape by climbing on the tree.
If (c1) happens you are in a dream;
If (c2) happens, that is just the fruit of your imagination
If (c3) happens, that is plain reality
Enjoy your experiment!
(a tiger instead of a lion will also work)