Short explanation: liter is a measure of volume, the amount of space filled

I Lt of mercury weighs much more than 1 Lt of water even though they occupy same amount of space.

1 lt of water, for example, weighs more than 1 lt of cooking oil.

Conversely... 1 Kg of water occupies LESS space than 1 Kg of cooking oil but 1 Kg of mercury occupies much less space than 1 Kg of water.

ELI5