Any object can be integrated into pretend play as a representation of another object. This kind of representation is referred to as substitution. The ability to substitute one object for another emerges when a child is about 2 years old. In early instances of substitution, children are only capable of substituting objects that either have a similar structure or a similar function. For example, a child can pretend that a pen is a toothbrush, or that a television remote is a telephone. A pen has roughly the same shape as a toothbrush, while the remote has buttons that function similarly to a telephone. Around the age of 3, the child begins to master substitution and no longer needs physical or functional similarity between the actual object and the substitute. The child also becomes capable of substitution without the use of a concrete object, thus depending solely on imagination (e.g. putting their palm to their ear and having a conversation, indicating a phone call). The ability to hold more than one substitution at a time also increases, meaning that the child can pretend to be on the phone, walking a dog, and sipping juice all at once.