Just to add a little weight in numbers, billswift is right.
Also, if you heat something up, it will weigh more.
Even though the change in mass is very small, it can sometimes be measured. When an electron and a proton combine to form a hydrogen atom, energy is given off, so the mass of a hydrogen atom be less than the sum of the mass of the proton and electron. Physicists can now measure the masses of these three things precisely enough that they can confirm that this is true.
Also, if you heat something up, it will weigh more.
Even though the change in mass is very small, it can sometimes be measured. When an electron and a proton combine to form a hydrogen atom, energy is given off, so the mass of a hydrogen atom be less than the sum of the mass of the proton and electron. Physicists can now measure the masses of these three things precisely enough that they can confirm that this is true.