1. Change in speed: This is the most common way we think about acceleration. When an object speeds up or slows down, it's accelerating. A car accelerating from a stop sign, a ball rolling down a hill, or a plane landing on a runway are all examples of acceleration due to a change in speed.

2. Change in direction: Even if an object maintains a constant speed, it can still accelerate if it changes direction. Think of a car going around a curve at a constant speed. Even though the car isn't speeding up or slowing down, its velocity is changing because its direction is changing. This change in velocity signifies acceleration.

It's important to remember that acceleration is a vector quantity, meaning it has both magnitude (how much) and direction. So, an object can accelerate by changing its speed, its direction, or both.