100 + 100 = 180 !

de la flotte !
Right it’s easy. just take 100ml of water and 100ml of alcohol. The alcohol must be as pure as possible. If you can’t get this at a pharmacy, vodka will do but as it is only 45°, results will be less spectacular.

de l'alcool !
une épronvette
A kitchen measure is perfect but chemistry graduated test tubes are more accurate of course.
une éprouvette
100 ml
<--- 100 mL of water

100 mL of alcohol--->
100 ml
on mélange
Finally you  pour the contents of the first test tube into the second.
 No you’re not seeing things there’s less than 200 ml.

180 mL !

Buzzy explains

Well uh, what shall i say ? Let’s see. if you take a bucket of sand and mix it with a bucket of marbles, the volume is not exactly equal to the sum of the volumes of both buckets. What ? you think it’s normal ?
good. this simple explanation means I can hide my ignorance in this matter. Molecules don’t behave in the same way as marbles or grains of sand. If perchance a reader has a more satisfying explanation to put for ward, please let me have it !