A simple but effective introduction to the concept of algorithms. Students are shown a series of random numbers one at a time, and have to tell the teacher which is the largest number.
They don't know the range, and they don't know the quantity of numbers. However, all students can do it, by quickly creating a set of simple rules to follow in their head - an algorithm. Without realising it they create a set of instructions to follow, they create a variable, and they carry out comparison tests. They use input, and output, and effectively demonstrate all of the attributes of a simple algorithm.
This demonstration can then be used to introduce these ideas, and the whole concept of computational thinking.