In one or two sentences, explain why the combined weight of several liquids mixed in an open flask can be less than but never more than the sum of all the weights of the ingredients.

make it sound like a ten year old wrote it

When you mix liquids together, they can smoosh really close and take up less space and sometimes even make bubbles that go poof into the air, so the mix can weigh less than all the stuff you put in, but it will never weigh more 'cause nothing extra is getting in there.