A referee is a person who makes sure the players follow the rules at games such as football and basketball.
A sentry is someone who stands guard over something, to protect it. The sentry in this picture is guarding the entry to the Tower of London, which contains the Queen's jewels.
Something is slippery if it very hard to get a hold of, or you can't stick to it. A slippery surface is one your shoes or feet don't grip onto, so you fall over when you are walking on them. Slippery things are usually very smooth and are often wet and slimey.
Some things that are slippery are ice, wet roads, banana peels, and rocks at the beach at low tide that have algae growing on them.
The opposite of slippery is sticky.
Surgery is a means of making people better by cutting into them and fixing what is wrong with them inside. Someone who performs surgery is a surgeon.
