Here's why:

* High Altitude: Glaciers form in high altitudes where temperatures are consistently below freezing.

* Years of Snowfall: Snow accumulates over many years, compressing and transforming into ice.

* Never Completely Melt: Glaciers are constantly losing ice through melting and sublimation (ice turning directly into vapor), but they also gain ice through snowfall. The balance between these processes determines whether a glacier advances or retreats.

Let me know if you have other questions about glaciers!