$1$ heap of sand $+\ 1$ heap of sand $= 1$ heap of sand? My uncle, who barely passed elementary school math (which leads me to believe he read this in some kind of joke magazine), once told me this when I was very young. 
$$1 \text{ heap of sand } + 1\text{ heap of sand } = 1\text{ heap of sand}
.$$
It does sound like a joke, but as I learned more and more math (still at basic college math), I still couldn't (and can't) disprove/prove it. 
So we all know this is true "linguistically," since if you add 1 heap of sand to another one, it will still be a heap of sand, but mathematically, 1 + 1 cannot equal 1.
I expect there to be, and wouldn't be surprised if there weren't, similar questions to this one but I couldn't find it using the search feature.
I guess I'm going out on a limb here because of the stupidity (and perhaps the obvious answer) of this question, but I seriously cannot disprove this based on what I know. 

One thing you must notice here is that:

$$1\, \text{heap of sand} + 1\, \text{heap of sand} = 1\, \text{heap
 of sand} \not \large\Rightarrow 1+1=1$$

The reason is that $1$ heap is not well-defined or quantitative or mathematical enough for being used to perform mathematical operations like addition,subtraction and make equations. If you had mentioned $1$ Kg. or perhaps $1$ quintal or even, in some special case, $1$ bag of sand, then also you can perform addition,subtraction etc. and equate both sides of the "$=$" sign. Otherwise this equation makes no sense.
It is similarly wrong to say that $$1\, \text{straight line} + 1\, \text{straight line} = 1\, \text{straight line} \implies 1+1=1$$
Instead as pointed out by McFry, you should try to use set theory to clarify your problem since it is a clear case of composition.
