Transubstantiation is a philosophical concept to explain what happens during Mass. The general idea is that everything in nature has a substance that doesn’t change, and attributes (called accidents, but I think attributes may be a bit clearer in modern language) that do change. The substance of a chair is its “chair-ness”, its attribute could be “green”. You can change the attributes, and it would still be a chair. If you change the substance, it is no longer a chair.
The great mystery of the Mass is that the bread and wine do not change attributes, so they still feel, smell, taste like bread and wine, but their substance does change. From “bread-ness” and “wine-ness” they change to “Body-ness” and “Blood-ness”. They do become the Body and Blood of Christ.
The interesting thing is that many people find it strange that bread and wine become Body and Blood, but fewer mention the even stranger fact that the attributes stay the same. Yet, that is what it is.
If you look at the miracle at the wedding in Cana you will see that water not only changes into the substance of wine, it has all the attributes of wine. Still a miracle, but not as miraculous as the Mass.
One of the essential aspects of a miracle is that they are not “natural”. I think it would be hard to find an example of transubstantiation in nature. But there is another reason for this: our senses notice attributes, not substance. We do not see “chair-ness”, our mind builds the knowledge of “chair-ness” from the attributes of chairs we see in our life, and from logic, reasoning. Therefore it would be practically impossible for us to notice any form of transubstantiation, as the attributes don’t change.
You can see this in Mass. There is absolutely no way to sense the difference between the bread and wine before, or the Body and Blood after. As Catholics we care a lot about this, so we make sure all Blood is consumed, including any particles left in the chalice (this is done at purification). All particles of the Body are also consumed, or left in the tabernacle. We take very good care that it is impossible to treat the Body as if it were bread. Because you just cannot tell otherwise.