Typically, a sty produces a bump on one of the eyelid at the site of the gland that is blocked. Symptoms and signs such as

redness,
pain,
tenderness usually accompany the lump on the eyelid.

It may be painful to blink the eye, and excess watering or tearing can result. In some cases, there is a small, yellow spot at the center of the bump; this is a collection of pus.

Causes of a sty

There are two types of sties, hordeolum and chalazion. A hordeolum is a blockage of a sweat gland on the skin of the lid and base of the eyelashes, or one of the small sebaceous glands found at the base of the eyelashes. A chalazion is a blockage of a meibomian gland, a special sebaceous gland unique to the eyelids. These glands are located in a single row in each lid.

Blockages of either of these gland types happen if the gland's opening is obstructed for any reason. Common causes of the blockage are scar tissue, foreign substances like makeup or dust, or if there are thickened secretions, causing the material from the gland to flow slowly or not at all.