I feel as though I have no choice but to be a skeptic about our ability to know the world on the sense experience given the information that is being presented. Our senses are touching, hearing, smelling and tasting, I believe it is quite possible that a person could think they see, touch, and smell something such as a glass of bear but there be no glass of beer present, therefore their perception of this glass of beer is false. There is a good possibility that this person is suffering from any of the numerous possible sensations, auditory, visual or tactile, experienced without external stimulus and caused by mental derangement, intoxication or fever, in other words this person could be hallucinating.
There are many ways that the senses can be tricked into believing things that are not true, an example is when a person takes the drug LSD, this drug is one which alters the state of the mind and tricks it into visually perceiving things which are not real such as pink elephants, green rats, gold skin and so on.
Hallucinations may occur when pressure is applied to different sections, drawing different reactions from the person being affected, these reactions are caused by the affected person seeing things which they perceive to be real . Hallucinations are only one way by which the visual perception of an object can be altered there are many more ways by which the visual perception of an object can be altered; for example consider a square envelope, pay very close attention to what you see when you look at this object. If the envelope does not move but you do then your perception of this object will continually change as you move about and the 'square envelope' no longer looks square.